There are two issues: sound and reliability. I've found that most Class A amps running over 100 watts, need a LONG warm up time (like a day or two). Class A/B and B amps don't seem to need this kind of warm up (neither do low powered class A amps). These can generally warm up in an hour or two. I have found some amps that do not follow this so well. I have a Classe Seventy that sounds great after only 1 hour of warm up, but I understand the Classe DR series (small wattage) needed very long warm up times--they also ran very hot. So you need to determine what sonic improvements you get over the warm up time. The second issue is reliability. I listen to music a couple of hours a day. I don't want to cycle the boards through the heating cycles especially on HOT amps like Krell and Levinson every day (I leave them on for sound improvement as well--but even if that wasn't an issue I would still leave them on for reliability). The only time they are turned off is if I am out of town for any length of time. The amps I use for HT only get used about 2 or maybe 3 times a week, so I only turn those on when I'm going to watch a movie. I only let them warm up about 15 minutes--which is probably less than they really need, but it's for home theater and I really don't think I can tell the difference for that application.
