I can't speak for any other brands, but the Bose warns you with a fast blinking Amber light when you are down to 8 hours battery remaining and from 2 hours remaining turns to a flashing red light. There is absolutely no excuse for running out of battery whilst airborne!
Some Bose were sent out with a faulty battery compartment that was draining a lot of battery life leaving you with around an hours worth of usage. They are in the process of sending out new ones to the affected Bose users; so if you were unaware of this and you went without batteries, just because the light wasn't blinking, you'd have to put up no ANR for the rest of the flight.