Hi all,
The caveat here is tube power amplifiers. Shut them down at the end of your listening session.
The good news is that tube power amps come to life in 20 to 60 minutes, with the longer number being the exception (e.g. when you have Black Gate power supply capacitors).
This 20 minute number (rough guideline) is consistent with other parts of your system waking up - especially the transducers - notably your phono cartridge, drivers in your speakers, but also things like your turntable bearing lube.
Back when I ran s-s amps, most of them took 72 to 96 hours to stabilize. The achieved most of their "sound" in the first 24 hours, but the 96 hour rule of thumb applied in terms of the last bit of warm-up.
With what used to be daily thunder storm activity here during our Colorado Summers, my wife would take to unplugging computers and hi-fi gear at the first sign of heavy weather (I don't mean the album by that name ).
This meant that from June through August, my gear was never fully warmed up - operating perhaps 18 hours per day with a 6 hour shutdown.
Parenthetically, I've not found any well designed line level (or RIIA stage) tube component which benefit from more than an hour's warm-up. They may exist, but in general, the snap to life fairly quickly too.
One other point to consider is that of safety.
Now, in this day and age, there are quite a few devices in our houses which run 24x7, and I don't mean to make you paranoid over the following statement, but you might think in terms of not burning your house down.
Many people in offices leave their computers on 24x7 (along with their monitors - this latter thing puzzling me). Well, I was working late one evening and was fortunately around to unplug someone's monitor which begin to billow plumes of smoke - likely from an electrolytic p-s cap failing.
Cheers,
Thom @ Galibier