Most preamps ought to stay on 24/7. Mine have no power switch (solid state). Eve tubes, if they run moderatlely cool to warm are best just left one. There is some sonic benefit and the power drain/heat is minimal in many preamps.
Power amps are anther story. I’ll steer clear of tubes where the tube itself and state of bias is critical to how much it may be left on and if you can even get replacements!
For solid state power amps, it depends on the design. Without getting techie some have standby modes that allow them to warm up in a claimed 15-60 minutes. There is no danger to leaving them on - just heat, and wasted electricity -- which is both a cost and an enviro factor.
The real answer on how much warm up can be best determined by your ears. If you cannot hear the difference reliably between 30 and 120 minutes, then who cares? turn them off. Yes, as noted, there is a surge at turn on but all most stuff is 30 year old prototypes and has been turned on and off thousands of times. QED. For me, 30-60 minutes n the power amp (from **standby**) is sufficient. From truly off, several hours. That’s why i designed standby!
I have had owners and dealers claim a subtle improvement over days. Maybe its true, but it is also very much at the margin and likely only audible by those with excellent hearing, great systems and/or terrific imagination.
I really don’t believe that there is a single answer, given the many variables. And the difference between waking from standby and truly powering up, and stabilizing is just huge. In my case you can hear the image expand, the bass extend, and the top recede. Its very dramatic ( and kinda cool).
G
