depends on tag: but in general I would say that fulfilling following would be good enough: (1) lack of more prominent schema and (2) 1000 items being mapped or 90% of all known items worldwide being mapped (3) multiple people using this tag.
For items applying very often or rare values of popular tags the threshold can be higher (dual_carriageway=yes is not “de facto” tag as tiny part of places is using it), but it does not apply here.
