My experience is that if flying to/from/in the USA the seat belt sign stays on for far longer than necessary; if flying elsewhere then it isn't as much of an issue.
I have, however, discovered the exception to this rule on a recent return flight from AKL to LHR on Air China - the seat belt sign simply doesn't get switched off at all from boarding until disembarkation. This was the case on all 4 flights so I assume is their standard practice.