Question about valves running hours

Thank you all for your comments. 
My amplifier is a push-pull from Cayin. 
When I bought it, the lady in the shop told me to run it for periods no longer than 7 to 8 hours due to the heat. And to wait some time before turning it on again (I understood waiting around one hour or too, but maybe some minutes are enough).
I couldn't ask for more details because her English was poor and I don't speak Chinese! I'm living in China...
So now I'm just trying to get your thoughts about this matter to better understand my amp. 

Williewonka, sorry for your bad experience with that Chinese brand, but Cayin is a well known reputable name. Their products are top notch with lots of good reviews all over the world. Doesn't mean that a product coming out of their shelves couldn't be defective, but lets assume my amplifier is all good.
I guess all of us agree that the major issue when running valves for a long long time (more than 8 hours) it's the excessive heat. And heat is what shortens the life of a valve.
As for having to wait some minutes prior to turn it On again, yesterday someone told me that it is important because of the capacitors (?). It seems they have to "discharge" or dissipate their energy before we connect the amplifier again.
Anyone with a good simple explanation about this? 

Another pertinent question: If valves were the power support to all communications in a recent past (from tv and radio broadcast to military coms), for how long would they live? Would a valve survive more than a few hundred hours? Were they replaced many times a year? Or did the equipments have different banks of valves to switch between them? I'm curious about how people used to operate the tubes 50 years ago.

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.