Question about valves running hours


I'm new to the tubes world and when I bought my amplifier the seller told me not to run it for more than 8 hours, letting it for rest a couple hours before the next listening session. I wasn't told why, but now I got curious and I want to know more about it. 
What's the reason for this limitation? What can we damage and why?
128x128migueca
" On/off cycling during listening sessions is ridiculous".
It in fact is unless the listening session is way too long as mentioned 8 or more hours. Give yourself AND amp rest for at least few hours.

I don't believe the OP was talking about on/off cycling during a particular listening session.  But resting the equipment between listening sessions. People are different and have different schedules.

I am not retired and work during the day.  When I get home and feel the need to sit and listen and enjoy music, I turn my system on about an hour or so before listening and when done (because I have to go to bed and sleep to get ready for work the next day), I turn the system off.  I think this is mostly true for most people.

Retirees on the other hand, may listen for a few hours, leave to run some errands or some such and come back home and want to start listening again.

No big deal.  I would still turn my equipment off anyway.  Modern equipment should not have any on/off cycling and current inrush issues because they have inrush limiters or slow start circuits that limit inrush.  Also, some electronics don't have circuits that drain capacitors when off, so many times the capacitors still have a charge on them anyway after a few hours, so again, there is limited inrush current.

The only cycling issue would be the switch itself and maybe the bridge diodes.  But, I wouldn't count on that either.

If you are going to leave for a few hours, I would turn the equipment off.  If it is solid state, (with exception of class A amps), there shouldn't be a problem leaving them on.  Like I said, personally, I would turn it off anyway when I leave.  I don't like having electronics on when I'm not home.  Refrigerator notwithstanding.

But, leaving tubed equipment on that don't have standby circuitry is simply asking to replace the tubes much earlier than necessary.

enjoy

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. 
That makes no sence at sll. I recently bought the Excellent Jolida 
Fusion 3602  integraded amp.with a bunch of of upgraded parts including the Excellent 
Rike Audio paper oil capacitors that take 300 hours to break in.Check out their Amplifiers  $300,000+.
I have used Vacuum tube gear for years as of today I have been running this Integrated amplifier one week straight 24-7 with 
Vintage  output and phase splitter tubes, the second week changed out the EL-34 tubes  input tubes after 175 hours ,and installed KT- 150 Vacuum tubes just flipped a switch on back. Factory stock tubes are guaranteed for a minimum of 1,000 hours of play time.Vintage tubes many times that .
If your amp is built correctly it should have a stand by circuit, as well as a slow start so a surge  of voltage doesn't shock the tubes on start up.
Also manual bias of the larger input tubes is much more accurate 
Way of biasing the individual tubes ,auto biasing has a bunch of resistors to average the circuit which in turn takes away a bit from the accuracy a bit overall.this several engineers Have said as a purist approach. 
My work system is driven by a 55 year old Masterwork amp (by pilot).  I use it for 8 hour work days all of the time and it never fails me.  It does get hot but I make sure it's well ventilated.  If this amp can take the abuse without ever burning up tubes or caps, why can't a new one?