That’s the thing: it can be arbitrarily far away. I think this will also depend on the design of the quest. If you ask someone whether this street has a signposted speed limit, they might say no, because you can only get to the street after passing a speed limit sign on some other street. That still applies, because a speed limit doesn’t automatically reset every time you make a right turn.
no:traffic_sign=maxspeed (or no_traffic_sign:maxspeed=yes if you like) would simply mean that no speed limit sign is physically present along the roadway, even if one farther away applies to the roadway. This information would be of no use to data consumers, but it would tell another mapper that they shouldn’t distrust the maxspeed=* just because they see no sign on the spot.