When do you turn off the tubes?

Having had recent surgery and spending a lot of time at home, I was wondering how long a break from music do you feel you need to be taking before turning off your tube amplifier.I could leave it on all day even when not listening but that would be 12+hours/day.While my PrimaLuna Dialogue HP is gentle on tubes, the thought of retubing 8 KT150s is an expensive prospect.I usually turn the amplifier off if I am not going to listen for around 3 hours.What do other people do?