I leave my amps on 24/7 unless there is an electrical storm, then I unplug 'em. Like Rsuminsby, I have had Adcoms for many years, and it takes about 45min for the sound to noticeably change, but it really takes a day or so before they fully stabilize (the new MOSFET ones take a bit longer). The reason to leave a Solid State amp on is so you don't have to wait for it to warm up, but keeping the amp on also stabilizes the bias. When an amp is first turned on, the bias is high and needs to settle with the ambient temperature. As the amp warms up, the ambient temperature rises, which, in turn, lowers the bias. If you can imagine a scale or a see-saw leveling off, you get the idea. This bias settling takes quite a while, so I find a day is usually good. Once you play music and get the amp hotter, the settling time becomes a bit faster, because the transistors are operating in their quiescent range.
