Was on a 9 hr flight last week (MNL-AUH), and the seatbelt sign was on for the whole flight, with only the slightest hint of turbulence for about 5 minutes mid flight. After an hour or so, it was pretty much ignored by those needing the toilet. On the other hand, outbound a few days earlier (AMS-MNL) with a different airline, and with a fairly constant judder of light turbulence throughout the flight, never saw the seatbelt sign once.
Wondered in the first situation whether it's just a precaution and sort of a legal 'cover my back' seatbelt sign. Either way I don't care as I just sleep and press the call bell for water if i desire any rather then getting up on the assumption that if its an inappropriate time, I'll be told no. It isn't my responsibility as a passenger to worry about the crews rotation and rest during the flight, I have every confidence they are professional enough to deal with that themselves.