Question about valves running hours

Whart -  thank you for that "History class" :) It's really interesting to learn how people dealt with gear in the past - this case, valves.
It seems a valve was something simple and common, like a light bulb.

My pleasure. Obviously, that was the States. Other countries, practices varied, I would assume. (Hey, in London they still have a store for barrister wigs and a store devoted to umbrellas if I recall). I'm also pretty confident that the big tube Vlad Lamm uses in his SET amps was long used by the Soviet Union as part of the guidance system for their ICBM (nuclear missiles). It's a pretty robust tube! 

The heat is develop when the power tube plate dissipates,the more watts the more heat and the more heat the  shorter the tube life.A well designed amp should have about 60-70% of total plate dissipation

If the amp is biased correctly it can play long periods of time without issue.  I listen to my amps for hours , I do turn them off when I go out though, but cycling the power for short periods actually shortens tube life

Tubes are like incandescent lamp bulbs. They wear out in time, and switching them off between auditions helps lengthen their lifespan. One set of tubes will last for let's say, two to three years (daily use) without losing the sparkle in the treble range, and I'm talking about audiophile-quality use here. Their turning on and off causes temperature changes to the internal elements and this is what shortens their life. It's advisable to turn them some time before listening and letting them warm up to music. An ageing tube loses the top end, then the mids and then the bass, in this order.