I just used GraphHopper to analyze the usage of the duration tag for OSM ways tagged as route=ferry. The duration tag is missing in many cases (~87%), which somewhat makes sense for short ferries (like river crossings), but for longer distance ferries probably in most cases it should be possible to find out information about the ferry duration online. I created this list of ferry routes >500km that do not provide the duration tag (as of March 9th, 2022):
way: 28328718, distance: 581 km, link: [http://openstreetmap.org/way/28328718](http://openstreetmap.org/way/28328718)
way: 46593843, distance: 534 km, link: [http://openstreetmap.org/way/46593843](http://openstreetmap.org/way/46593843)
way: 70108523, distance: 1058 km, link: [http://openstreetmap.org/way/70108523](http://openstreetmap.org/way/70108523)
way: 72981466, distance: 602 km, link: [http://openstreetmap.org/way/72981466](http://openstreetmap.org/way/72981466)
way: 72981468, distance: 783 km, link: [http://openstreetmap.org/way/72981468](http://openstreetmap.org/way/72981468)
way: 72981470, distance: 583 km, link: [http://openstreetmap.org/way/72981470](http://openstreetmap.org/way/72981470)
way: 72981474, distance: 595 km, link: [http://openstreetmap.org/way/72981474](http://openstreetmap.org/way/72981474)
way: 74313291, distance: 540 km, link: [http://openstreetmap.org/way/74313291](http://openstreetmap.org/way/74313291)
way: 84517904, distance: 1408 km, link: [http://openstreetmap.org/way/84517904](http://openstreetmap.org/way/84517904)
way: 84517909, distance: 1407 km, link: [http://openstreetmap.org/way/84517909](http://openstreetmap.org/way/84517909)
way: 84522557, distance: 1287 km, link: [http://openstreetmap.org/way/84522557](http://openstreetmap.org/way/84522557)
way: 89607195, distance: 664 km, link: [http://openstreetmap.org/way/89607195](http://openstreetmap.org/way/89607195)
way: 130629215, distance: 613 km, link: [http://openstreetmap.org/way/130629215](http://openstreetmap.org/way/130629215)
way: 130629216, distance: 1177 km, link: [http://openstreetmap.org/way/130629216](http://openstreetmap.org/way/130629216)
way: 131893728, distance: 526 km, link: [http://openstreetmap.org/way/131893728](http://openstreetmap.org/way/131893728)
way: 154560626, distance: 528 km, link: [http://openstreetmap.org/way/154560626](http://openstreetmap.org/way/154560626)
way: 164921795, distance: 1007 km, link: [http://openstreetmap.org/way/164921795](http://openstreetmap.org/way/164921795)
way: 181327424, distance: 827 km, link: [http://openstreetmap.org/way/181327424](http://openstreetmap.org/way/181327424)
way: 229413306, distance: 554 km, link: [http://openstreetmap.org/way/229413306](http://openstreetmap.org/way/229413306)
way: 229499873, distance: 714 km, link: [http://openstreetmap.org/way/229499873](http://openstreetmap.org/way/229499873)
way: 231003224, distance: 513 km, link: [http://openstreetmap.org/way/231003224](http://openstreetmap.org/way/231003224)
way: 233682992, distance: 1698 km, link: [http://openstreetmap.org/way/233682992](http://openstreetmap.org/way/233682992)
way: 255920387, distance: 654 km, link: [http://openstreetmap.org/way/255920387](http://openstreetmap.org/way/255920387)
way: 256301211, distance: 735 km, link: [http://openstreetmap.org/way/256301211](http://openstreetmap.org/way/256301211)
way: 256772904, distance: 788 km, link: [http://openstreetmap.org/way/256772904](http://openstreetmap.org/way/256772904)
way: 274285415, distance: 936 km, link: [http://openstreetmap.org/way/274285415](http://openstreetmap.org/way/274285415)
way: 309939688, distance: 526 km, link: [http://openstreetmap.org/way/309939688](http://openstreetmap.org/way/309939688)
way: 358398272, distance: 579 km, link: [http://openstreetmap.org/way/358398272](http://openstreetmap.org/way/358398272)
way: 358574084, distance: 532 km, link: [http://openstreetmap.org/way/358574084](http://openstreetmap.org/way/358574084)
way: 361348335, distance: 544 km, link: [http://openstreetmap.org/way/361348335](http://openstreetmap.org/way/361348335)
way: 404384710, distance: 733 km, link: [http://openstreetmap.org/way/404384710](http://openstreetmap.org/way/404384710)
way: 404385815, distance: 817 km, link: [http://openstreetmap.org/way/404385815](http://openstreetmap.org/way/404385815)
way: 404414835, distance: 530 km, link: [http://openstreetmap.org/way/404414835](http://openstreetmap.org/way/404414835)
way: 404414838, distance: 564 km, link: [http://openstreetmap.org/way/404414838](http://openstreetmap.org/way/404414838)
way: 404414841, distance: 986 km, link: [http://openstreetmap.org/way/404414841](http://openstreetmap.org/way/404414841)
way: 423867828, distance: 741 km, link: [http://openstreetmap.org/way/423867828](http://openstreetmap.org/way/423867828)
way: 461640745, distance: 793 km, link: [http://openstreetmap.org/way/461640745](http://openstreetmap.org/way/461640745)
way: 464181826, distance: 1168 km, link: [http://openstreetmap.org/way/464181826](http://openstreetmap.org/way/464181826)
way: 495298842, distance: 523 km, link: [http://openstreetmap.org/way/495298842](http://openstreetmap.org/way/495298842)
way: 505318747, distance: 619 km, link: [http://openstreetmap.org/way/505318747](http://openstreetmap.org/way/505318747)
way: 505318748, distance: 1098 km, link: [http://openstreetmap.org/way/505318748](http://openstreetmap.org/way/505318748)
way: 505318753, distance: 645 km, link: [http://openstreetmap.org/way/505318753](http://openstreetmap.org/way/505318753)
way: 550202121, distance: 511 km, link: [http://openstreetmap.org/way/550202121](http://openstreetmap.org/way/550202121)
way: 550203352, distance: 620 km, link: [http://openstreetmap.org/way/550203352](http://openstreetmap.org/way/550203352)
way: 550415117, distance: 699 km, link: [http://openstreetmap.org/way/550415117](http://openstreetmap.org/way/550415117)
way: 550415128, distance: 946 km, link: [http://openstreetmap.org/way/550415128](http://openstreetmap.org/way/550415128)
way: 592194702, distance: 583 km, link: [http://openstreetmap.org/way/592194702](http://openstreetmap.org/way/592194702)
way: 730774574, distance: 505 km, link: [http://openstreetmap.org/way/730774574](http://openstreetmap.org/way/730774574)
way: 730957695, distance: 509 km, link: [http://openstreetmap.org/way/730957695](http://openstreetmap.org/way/730957695)
way: 743680878, distance: 1043 km, link: [http://openstreetmap.org/way/743680878](http://openstreetmap.org/way/743680878)
way: 753103069, distance: 738 km, link: [http://openstreetmap.org/way/753103069](http://openstreetmap.org/way/753103069)
way: 753103071, distance: 1140 km, link: [http://openstreetmap.org/way/753103071](http://openstreetmap.org/way/753103071)
way: 768415237, distance: 1508 km, link: [http://openstreetmap.org/way/768415237](http://openstreetmap.org/way/768415237)
way: 784811682, distance: 911 km, link: [http://openstreetmap.org/way/784811682](http://openstreetmap.org/way/784811682)
way: 784843320, distance: 596 km, link: [http://openstreetmap.org/way/784843320](http://openstreetmap.org/way/784843320)
way: 792805198, distance: 522 km, link: [http://openstreetmap.org/way/792805198](http://openstreetmap.org/way/792805198)
way: 801899839, distance: 745 km, link: [http://openstreetmap.org/way/801899839](http://openstreetmap.org/way/801899839)
way: 803749088, distance: 563 km, link: [http://openstreetmap.org/way/803749088](http://openstreetmap.org/way/803749088)
way: 817792853, distance: 1038 km, link: [http://openstreetmap.org/way/817792853](http://openstreetmap.org/way/817792853)
way: 817807715, distance: 816 km, link: [http://openstreetmap.org/way/817807715](http://openstreetmap.org/way/817807715)
way: 817807716, distance: 851 km, link: [http://openstreetmap.org/way/817807716](http://openstreetmap.org/way/817807716)
way: 817807718, distance: 1190 km, link: [http://openstreetmap.org/way/817807718](http://openstreetmap.org/way/817807718)
way: 837995827, distance: 996 km, link: [http://openstreetmap.org/way/837995827](http://openstreetmap.org/way/837995827)
way: 843046077, distance: 585 km, link: [http://openstreetmap.org/way/843046077](http://openstreetmap.org/way/843046077)
way: 965078020, distance: 684 km, link: [http://openstreetmap.org/way/965078020](http://openstreetmap.org/way/965078020)
way: 973754594, distance: 924 km, link: [http://openstreetmap.org/way/973754594](http://openstreetmap.org/way/973754594)
way: 976053779, distance: 528 km, link: [http://openstreetmap.org/way/976053779](http://openstreetmap.org/way/976053779)
way: 977915953, distance: 701 km, link: [http://openstreetmap.org/way/977915953](http://openstreetmap.org/way/977915953)
way: 979911973, distance: 759 km, link: [http://openstreetmap.org/way/979911973](http://openstreetmap.org/way/979911973)
way: 999670971, distance: 1652 km, link: [http://openstreetmap.org/way/999670971](http://openstreetmap.org/way/999670971)
way: 999675555, distance: 1683 km, link: [http://openstreetmap.org/way/999675555](http://openstreetmap.org/way/999675555)
way: 999679765, distance: 862 km, link: [http://openstreetmap.org/way/999679765](http://openstreetmap.org/way/999679765)
way: 1008735423, distance: 1377 km, link: [http://openstreetmap.org/way/1008735423](http://openstreetmap.org/way/1008735423)
way: 1008743106, distance: 650 km, link: [http://openstreetmap.org/way/1008743106](http://openstreetmap.org/way/1008743106)
way: 1012580086, distance: 634 km, link: [http://openstreetmap.org/way/1012580086](http://openstreetmap.org/way/1012580086)
way: 1013195522, distance: 939 km, link: [http://openstreetmap.org/way/1013195522](http://openstreetmap.org/way/1013195522)
way: 1014471989, distance: 1080 km, link: [http://openstreetmap.org/way/1014471989](http://openstreetmap.org/way/1014471989)
way: 1018302648, distance: 4881 km, link: [http://openstreetmap.org/way/1018302648](http://openstreetmap.org/way/1018302648)
way: 1018615710, distance: 959 km, link: [http://openstreetmap.org/way/1018615710](http://openstreetmap.org/way/1018615710)
way: 1018882060, distance: 805 km, link: [http://openstreetmap.org/way/1018882060](http://openstreetmap.org/way/1018882060)
way: 1020151414, distance: 913 km, link: [http://openstreetmap.org/way/1020151414](http://openstreetmap.org/way/1020151414)
way: 1020398546, distance: 720 km, link: [http://openstreetmap.org/way/1020398546](http://openstreetmap.org/way/1020398546)
way: 1020913180, distance: 977 km, link: [http://openstreetmap.org/way/1020913180](http://openstreetmap.org/way/1020913180)
way: 1020987155, distance: 923 km, link: [http://openstreetmap.org/way/1020987155](http://openstreetmap.org/way/1020987155)
way: 1028107538, distance: 566 km, link: [http://openstreetmap.org/way/1028107538](http://openstreetmap.org/way/1028107538)
way: 1028249451, distance: 664 km, link: [http://openstreetmap.org/way/1028249451](http://openstreetmap.org/way/1028249451)
way: 1029167337, distance: 741 km, link: [http://openstreetmap.org/way/1029167337](http://openstreetmap.org/way/1029167337)
way: 1029653579, distance: 1093 km, link: [http://openstreetmap.org/way/1029653579](http://openstreetmap.org/way/1029653579)
way: 1029680994, distance: 575 km, link: [http://openstreetmap.org/way/1029680994](http://openstreetmap.org/way/1029680994)
way: 1029687089, distance: 586 km, link: [http://openstreetmap.org/way/1029687089](http://openstreetmap.org/way/1029687089)
way: 1030164612, distance: 537 km, link: [http://openstreetmap.org/way/1030164612](http://openstreetmap.org/way/1030164612)
way: 1030432845, distance: 1081 km, link: [http://openstreetmap.org/way/1030432845](http://openstreetmap.org/way/1030432845)
way: 1030543836, distance: 566 km, link: [http://openstreetmap.org/way/1030543836](http://openstreetmap.org/way/1030543836)
way: 1031395806, distance: 537 km, link: [http://openstreetmap.org/way/1031395806](http://openstreetmap.org/way/1031395806)
way: 1031846730, distance: 547 km, link: [http://openstreetmap.org/way/1031846730](http://openstreetmap.org/way/1031846730)
way: 1031860549, distance: 570 km, link: [http://openstreetmap.org/way/1031860549](http://openstreetmap.org/way/1031860549)
way: 1032077797, distance: 657 km, link: [http://openstreetmap.org/way/1032077797](http://openstreetmap.org/way/1032077797)
way: 1032377337, distance: 783 km, link: [http://openstreetmap.org/way/1032377337](http://openstreetmap.org/way/1032377337)
way: 1032395769, distance: 1175 km, link: [http://openstreetmap.org/way/1032395769](http://openstreetmap.org/way/1032395769)
way: 1032647030, distance: 563 km, link: [http://openstreetmap.org/way/1032647030](http://openstreetmap.org/way/1032647030)
way: 1032709885, distance: 651 km, link: [http://openstreetmap.org/way/1032709885](http://openstreetmap.org/way/1032709885)
way: 1032724802, distance: 796 km, link: [http://openstreetmap.org/way/1032724802](http://openstreetmap.org/way/1032724802)
way: 1032725514, distance: 513 km, link: [http://openstreetmap.org/way/1032725514](http://openstreetmap.org/way/1032725514)
way: 1032743080, distance: 773 km, link: [http://openstreetmap.org/way/1032743080](http://openstreetmap.org/way/1032743080)
way: 1032746410, distance: 851 km, link: [http://openstreetmap.org/way/1032746410](http://openstreetmap.org/way/1032746410)
way: 1033064091, distance: 1728 km, link: [http://openstreetmap.org/way/1033064091](http://openstreetmap.org/way/1033064091)
way: 1033923633, distance: 831 km, link: [http://openstreetmap.org/way/1033923633](http://openstreetmap.org/way/1033923633)
way: 1033933092, distance: 503 km, link: [http://openstreetmap.org/way/1033933092](http://openstreetmap.org/way/1033933092)
way: 1033982527, distance: 573 km, link: [http://openstreetmap.org/way/1033982527](http://openstreetmap.org/way/1033982527)
way: 1034325278, distance: 643 km, link: [http://openstreetmap.org/way/1034325278](http://openstreetmap.org/way/1034325278)
Is there a better place to share this kind of information? You can find more details about my analysis here: https://github.com/graphhopper/graphhopper/issues/2532