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

I don’t think a question about preserving the tubes is neurotic at all. It’s a simple question which aims at good habits to preserve a resource.

Without over-thinking it, the better you match your listening time with the time the tubes are on, the longer they’ll last. That will give you the confidence to spend more on better tubes, rather than changing them out more often. If you find that estimating this is a pain, then just turn them on and forget the conservation thing; you'll get more pleasure that way.

Tube gear sounds better as it warms up and operates and so does solid state gear so the longer you listen and leave it on the better it will sound. Cartridges are especially like this too they usually require between 4 and 6 album sides to sound right especially a low output moving coil.

@wokeuptobose Past three tube amps both used and new, with new or NOS tubes -    

Startup:  
During initial burn in, with new tubes in my preamp and mono amps, they performed and sounded better after 45-60min of warmup.  All burned in now, right at 30 minutes or so they seem to start sounding pretty good. Takes less time now, maybe 20-30 min, transformers too.   

Shutdown:
If the tube amps and preamps are on, and I know I'm not going to sit back down and do any serious listening for the next 3-4 hours, I'll go ahead and just shut them off.

As others noted, prefer not to leave them unattended for too long.   

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!