Warm-up time for amps and preamps

Everyone is basically correct here. Solid State equipment usually takes about 30-60 minutes to get warm and stable. Electrolytic caps don't like heat but like being charged all the time. Tube filaments should always be on for a at least 30s before HV is applied. Using tube rectifiers automatically fixes this issue as they don't put out any HV until they themselves are warm. This BTW does not only apply to HiFi equipment but also to e.g. microwave ovens and radar transmitters. They need about 5 minutes to get warm since the magnetron is a big solid lump of metal. The ideal solution for tubes would be to have a low voltage pre-heat applied to the filaments all the time that keeps them warm but not glowing. 

So what should one do? Leave equipment on or switch them off? Well, It comes down to for how long time it will not be in use, the amount of energy it consumes when idle vs in stand-by or off and if you want to wait for it to warm up...

keeping your equipment on 24/7 is the best way I know of to decrease the life expectancy of your equipment, big time!  Heat is the killer of electronic components.  Heat is generated by tubes, high wattage resistors, high current output transistors etc.  This is why fans / heat sinks etc. are incorporated.  If you have ever looked at a PCB with high wattage resistors attached you will see darken areas where they are, plus cold solder joints forming.

The electrolyte paste in power capacitors, will dry out if just setting on a shelf or in equipment not being utilized.  Sometimes you can bring a electrolytic back to life by reforming it with a voltage source that is regulated.

Bottom line , to turn your equipment off when not listening is the best service you can do to your equipment.  But don't just let it set for months on end without operation.

All audio equipment electronically speaking, will be up to full reproductive audio response after a minimum of ten minutes.  Resistors, capacitors, diodes, solid state devices, etc. do not have a warm up period or break in time.  The only component that can benefit from time on is a valve tube.  because of the electrons boiling off the filament, increase conductivity with longer on time.

Tube equipment will begin to degrade over time do in part to an oxide coating that develops on the filament, not allowing for full conduction of the electrons to the grid and plate of the tube.

There are many "authoritative" answers here.
i wonder which ones are right.

The designer of my tube integrated said that it's fully warm and stable after 15 minutes.  I'll give it at least 5 before starting any playback, and 15+ before sitting down for a listen.

My DAC/Pre (no tubes) stays on 24/7.

My office system has a Yamaha 2-ch receiver I bought in 1996.  I've only turned it off about seven times, ever -- and it still sings. 

I believe rvpiano said it best. There’s been lots of threads about this already. One member mentioned awhile ago that a SACD player needs to be on for about 2-3 days to sound its best. Others have stated that SS gear needs to be on 24/7 unless not listened to for several days to a week. As much as I listen, I leave it all on 24/7. Everything is barely warm, at best. In fact, I can hardly feel the difference.

All the best,
Nonoise