When do you turn off the tubes?

Shut down when not listening. Never leave on when not home. Simple. 

Many tube amps have a stand-by mode in which the output tubes are off but the amp is still warm. My Cary amp have this. When I'm home and know I will be listening that day they stay in standby. When I leave the house or go to bed, everything gets turned off.
I once saw my tube preamp go up in smoke while it was just idling.

I usually shut the amps off each time I am done listening.  Whenever I leave them on thinking I am going to go back to listening, my day usually changes so I can't.  If I turn them off, the odds of being able to listen more dramatically increases.  And, these things get hot-- including heating up the room when you have 8 kt120's per amp x 2, a couple of m8080's and a 6922 in the preamp, and the Sophia's in the CD power supply.  24 tubes total when everything is on-- that's too much electricity to waste if it's not in use.  And, finally, I have seen a tube fail-- right in my amp-- spectacular but scary.  I want to be around if/when that happens.

Less than 2 hours and I leave it on with the cd player on repeat with the volume turned down a bit. When I'm ready to listen I just turn it back up and put a record on...

The most potential damage to your tubes is turning on your amp. The likelihood of a tube exploding and torching your house are tiny. Some amps will pop a fuse if a tube flares and some don’t.  Bias type is most of the question there. I’d be more concerned with being electrocuted frankly. I would turn off after an hour or two of idle. But only once a day. No we regarding tube pre’s leave te=hem on in standby mode except when on vacation. They draw minimal power,the tubes last eons and they stay heated. But remember to pout in standby. Caps, etc go bad first from sitting and second from wear. If you are worried about tube replacement then you should, well...hey it’s part of maintenance. And I’ve been guilty of going out of town leaving a pair of monoblocks on—had to call a friend to go and turn them ooff for me. Fortunately he could get in through my garage with the code. Enjoy!