rule of thumb: how long do you leave a tube preamp on versus turning it off then back on?


I just upgraded to an Audio Research REF 10 with 12 tubes and I am wondering how to best preserve the tubes. How long sitting turned on is equal to the wear and tear of turning it off then on again? I had a Rogue RP 7 with just four tubes so I didn't worry about this issue so much, I just turned it on when I began to listen then off at the end of the evening. Now I'm wondering where the sweet spot is? Away for a hour, two, three, turn it off when? Am I over-thinking this issue?
wokeuptobose
Use and enjoy what you paid for, you knew that tubes wear out.
Just power them off at end of each session. It is not wise to leave them on and be away for some time. Normally a tube will last longer in hours if not switched on and off but a year has approx 9000hrs so you will need replacements at least twice a year if left on all the time. Otherwise with normal use replacement would take place every 2-3 years.

G

Best not to leave powered up tube gear unattended


important advice, to be heeded, by @noromance -- better safe than sorry - even for preamps, i have seen tube rectifiers go up in smoke
after burn in (Brent Jesse advised 60 hours on, no signal needed)

then: on, 20 minutes, magic, off. preamp and amp 
Good grief! You are fortunate to own this level of gear-enjoy it. Turn it off whenever you’re done, turn on when ready to listen again...simple.

Cut back on reading audio forums to reduce neurosis.