Question about valves running hours

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.

The owner's manual for my tube amplifier advises that to increase tube life that it not be left running for more than an hour if it is not being used. Along the same line, a comment made in the owner's manual for my (tube-based) media player is that unlike solid state players tube output stage players should be shut down when not being used.  This will greatly prolong tube life.

Migueca,If the input voltage is not the  same as in the US,It affect the filament voltage and it's not 6.3 volts anymore and that's why tubes have shorter life if they are not rated.