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

My setup is a bit less exotic, an AR SP20 pre amp with a Ref 75 running KT150's.
I turn it on and just start enjoying it. No pre-listening warm-up. After about 30 minutes, it sounds noticeably better.
When I am done with my listening session, I turn it off, pre amp first, then the power amp. 
When starting it up, I reverse that sequence and first turn on the amp. Then the pre amp. 
I have always been leary of letting tube gear running longer than the listening time, and am careful to make sure the same start-up and shut-down sequence is consistently followed. 
Hope this helps!

On my Vinni Rossi L2ise which uses Takatsuki TA-300B  tubes at $1,500 a pair in the pre-amp section (which runs tubes more gently than in most amps) I turn on an hour before use as I can tell the improvement in sound quality, then leave after session on if I will be listening again in a few hours. If wife is away and I will be listening off and on all day or weekend, then I leave on and can tell it gets even better after 24 hours.

Funny. I'm old enough to remember when TVs and radios only had tubes. We turned them on and off numerous times and left them on for hours at time. Never remember having a TV  or radio go bad due to a tube failure. Maybe a poor comparison but we are a neurotic group and tend to over think things a bit.

J.Chip

Some of these comments are mistaken.  All gear has an optimal operating temperature; it does not sound better after it reaches that temperature.  

I don’t have an answer to OP’s actual question, but I thought I would restate it since so far most have answered other questions instead of what they asked.

What duration of time left on is equivalent to the wear of start up for tubes?

If you leave the room for 5 minutes it clearly makes sense to leave your tube equipment on. However, if you are done for the night and going to bed and probably won’t be using your tube equipment for 12 hours it makes sense to shut it down. What about the times in between? Most days I can listen for a little while in the late afternoon and then again late at night. For that average 2 hour gap I wonder exactly what OP asked. Do I leave it on or shut it down? I usually turn off tubes but leave on SS. I’m sure it would be heavily tube and equipment dependent. Some tube types are more tolerant to startup and some equipment is kinder.