I leave ACDC on a good 1-3 hours while I am in here. The SS setups now are so efficient with minimal heat buildup in the backbox, I just don't worry about it anymore.
The B/W games I am a bit more strict on. I turn them on to play, and then leave them off. Some of the circuit components in there go 150-200 deg Farenheit; I don't like that kind of heat going for too long.