Warm-up time for amps and preamps

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

Leave it on for extended life in many instances. My Class D amp runs very cool so heat is not an issue. Small preamp signal tubes last longer on as studies and real life experience show. Turning the tubes on and off often reduces tube life faster than just leaving them on. As always it depends on the design with no blanket statements applying to all gear. 

I leave it all on, even my Lampizator stays on(read manual).  I turn them off if there is an electrical storm in the area or if I am leaving home for several days.  I've had some SS devices that take 3 days to really come alive and do what they are capable of.  Try it and hear for yourself, it won't hurt them.  If leaving it on is going to harm it, you don't want it anyway.