Hi High roller
Really depends on the types of tubes you have.
I run 300bs, and if I am leaving the house for what I think will be an hour(which is always more) I turn it off.
In contrast, my phono pre has 6dj8s which don't like being turned on and off, so when I flick this unit on, it stays on till I go to bed. I think tubes like 6sn7s could likely be left on 24hrs a day for 5 years before they needed replaced, so again it depends on the tube and its relative characteristics, cost etc.
I think question two is debateable re tube health. I would let the system stabilize(a couple to a few minutes. There is debate re sonic improvements and temperature of components. I don't want to go there since your question was for tube life. Power tubes heat up pretty quick. I also don't think tube amps are generally that fragile. Ie they are designed to be used, not only looked at.
Question 3: I would leave them plugged in and save wear and tear on the plug.
Cheers
Rob