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

Very interesting dilemma. 

I have a similar set up. 

I have the Ref 40th anniversary, similar to your Ref 10. 
My rule of thumb which has worked for me since I've owned it. 

1.  I turn it on after I turn on the rest of my system, but before the Boulder amp. 
2.  I turn it off 2nd, after the Boulder. 
3.  I turn it on about 1 hour before I venture to my listening room.  I turn it off after each session.  I usually listen every night or other night for about 1-3 hours. 
4.  I keep track of the hours with the tube counter.  At 2000, start thinking or planning on replacing the tubes.  
5.  Give it enough head room to breathe.  Do not stack.  

How do you like your Ref 10 btw?

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.