It seems to vary a lot and regardless what airline.

I guess it's up to the pilots personality. I believe they have a weather radar on the flight deck so it can also be that the pilot soon expects more turbulence and therefore leave the seatbelt sign on.

It seems pretty clear though that they don't want to use the switch too frequently. Once they make a decision to turn it on, it is on for at least 15-20 min even though no turbulence.