Depends on the design of the equipment. The issue with transistors is that they behave differently at different temperatures. This means you need to get any heatsinking up to temperature as well. In the case of the S300 in free air, this takes a couple of days at idle (rather less if you're using the amp of course) because the thermal mass is huge and the heatsinking is efficient. However, if you're giving the amp some stick then you'll get the internal device temperatures up pretty fast... no more than 30 mins or so. It is necessary to differentiate idle warmup versus use warmup. Idle warmup is slow (except with class A), whereas use warmup is much faster.