Tube life

I fall asleep to music; wake in the morning and flip the amp off. So I have two questions:

(1) I’m guessing my amp thus stays on 50 or so extra hours a week with no signal. Does this have any significant effect on tube life? (EL84s). (I normally replace them every 2-3 years; if after a week or so, I am not aware of any difference, I put the old ones back in, wait year or so and try again.)

(2) what obvious signs are there that tubes need replacement? (other than the method noted above, the growing urge to replace them for the helluvit, or, say, the time one of them spectacularly burst into flame around 1AM; [that was AWESOME! ] Turned out to be poor pin connections, which had been showing signs of that for months; can’t remember whether the tube survived. The amp is still going strong.)

You have a pretty good handle on the situation it seems to me. Your trick of comparing old to new tubes is what I do too. Can never predict life any more than you can predict spectacular fireworks. 2-3 years sounds about right to me. With a wall timer you might stretch that out to 5-10.
MC--  Thanks.  I thought about the wall timer, but that doesn't seem worth the bother and I was afraid the very suggestion of introducing such a thing might lead to vilification here!   2-3 years is about the time I get antsy and begin wondering how a set set of tubes would sound anyway.  So all's good.  I won't worry about it.

Corelli--  Ha!  I will definitely pay more attention to warning signs.   But it was truly spectacular, in terms of both sight and sound. There was no danger of my sleeping through it.
I don't leave any electronic gear running unless I am in the room. Especially tube gear. Also tube life is finite so I want to use every available hour. 
Those were great videos, particularly the second (for many many reasons!).  Thanks.   

I don't 'push' my system at all--small room, apartment, etc.   Putting all these together, yeah, I'll stay with my present, completely unscientific!, way of dealing with them:   every couple of years, I'll see if re-tubing helps what I'm hearing.    (Of course I could up-grade--but why? I can't imagine things sounding significantly better than they do now.)