Tube amp on/off: best practice?

My main system, with a tube integrated, is in my studio where I work 10-12 hours a day, interrupted by the occasional offsite meeting or errand.

What's the best practice/rule of thumb for powering it up and down as I duck in and out of the studio? Just leave it on all day?
How long do I need to be away for it to be better to power it down?

Right now I'm firing it up and cooling it down about 3x a day, running it a total of 6-8 hours a day.