Most pilots seem to wait until cruise altitude now, no idea why. You can just ask one of us to see how long until he/she turns it off.
Maybe post this question on the "ask a pilot" thread?
Good idea. @:-) The seat belt sign is generally "illuminated" (to use a favorite word

) for significantly longer periods of time in the U.S. than in Europe. More than once I've definitely noticed flight crew using it as a kind of restraint on the pax while they use the lav - worse, failing to turn it off afterwards - and I do sincerely resent that.