Leave tube amps always on?

Despite my question back to the aj523, I do agree with big-greg's & tvad'e No responses; I always switch my tube integrated amps, or tube preamp off when not in use. 

Per LTA's website " we estimate tube life for our amps to be in the 10,000-20,000 hour range."  Did they say what the tube life degradation from that level you'd experience if you turn it on and off as "normal" ? 

No.

Vacuum tubes have a limited life span when turned on, whether playing music or not.

This is because the cathode element (the one that glows to throw off all those electrons used to amplify the signal) slowly wears out when turned on. The lifespan is usually expressed in the number of hours the tube is on. How many hours depends on the tube and how it is used in the circuit -- some amps push a tube harder than others. Power output tubes have a shorter lifespan than the smaller tubes used in preamps. The brand of tube is also a factor.

There are 8,760 hours in a year. If you have a power tube with an anticipated lifespan of 4,000 hours, you'll likely be replacing the tube twice a year if you leave it on 24/7.
If you listen 8 hours a day and turn it off when not listening, you'll get over a year from the same tube. Listen 4 hours a day, an you'll likely get 3 years out of it.
Different people look at ongoing maintenance expenses differently. You might want to ask the manufacturer of your amp about the anticipated lifespan of the tubes as used in your particular amp.