Tuners can also stay on all the time. Mine does. I use a coax antenna so the stray noise should be very limited.
I agree that for sound quality, it is best to leave most gear powered up. This is particularly the case with solid state stuff which takes a long time to stabilize. As far as longevity is concerned, I don't think gear should be left on. It certainly is not the case with tube gear and not the case with solid state gear that runs hot when idling (biased high into Class A) -- heat is a killer of components.
Tuners and other low temperature, low power dissipation gear probably is not hurt much by being powered up all the time (aside from greater vulnerability to power surges/lightning strikes). My Dynalab tuner is actually on all of the time, even when the power switch is turned off (this only mutes the output and turns off the panel lighting). Dynalab apparently favors being powered up all the time. My CD player is also designed to be powered up constantly. My solid state preamp, a Levinson No. 32, is also designed to be on all of the time.
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.
Thom @ Galibier
Great advice all around. I found it interesting what you said about Black Gates. A dealer was telling me about a customer who had his amps upgraded with Black Gates. The customer complained that the amp sounded worse, and this made no sense because he liked the upgrade when he heard it at the dealer's shop. It turns out that the customer turns his amp on and listens for relatively short sessions and the dealer almost never turns off his amps. The customer switched back and was happier.
The safety issue is a frustrating one for me. I have a Naim CDP 555 cd player. The manufacturer's representative says it should be powered up all the time. Indeed, if it is turned off for more than a few minutes, it takes a really long time (several hours) to come on song again. The instruction manual, however, has all these dire warnings about lightning and protecting the gear by completely unplugging it.
I hope I didn't torture you with the safety issue.
We have a couple of receivers for our satellite TV Dish network) and leave them on 24x7. They're so fragile (#$%! cheap), that even though we hook them up to surge protectors, we've had random power-up failures ... hence the 24x7 operation.
I still think about lightning issues here, and don't know how to advise you on your Naim issue. I would likely power mine on 24x7 - unplugging it if I leave town and such.
Black Gates are funny. In one amp I played with them in, they would go through several stages of warm-up - beginning to improve about 20 minutes in, then, taking a step backward for several minutes (beginning to sound disjointed). Finally, 45 minutes to an hour in, they would lock in.
You know when you're in a listening session with an acute eared buddy and you're in conversation? Suddenly, you both clam up and look at each other. We would both notice the stages of warm-up on these caps (in these amps) at the same time.
Note that the above discussion relates to the large power supply filter caps. The make smaller caps which don't seem to exhibit this effect. In vacuum tube input / driver stages for amplifiers, the smaller Black Gates work very nicely in the cathode resistor bypass position. I