I have generally found UA, going back to CO, just kept it on almost always. They'll take it off once they reach cruising altitude for about 5 minutes before putting it back on. Almost like an indicator that you can now get up and about. But it's mostly on. I've seen similar on AA, but not DL.
On foreign airlines, it's basically off always accept when it has to be on (takeoff, landing, turbulence).
I've generally seen enforcement strict during takeoff, landing, turbulence, and special announcements as well as service, but lax at other points