I should clarify that my system is on a lot when I am home, and I work out of my house, so I'm home a lot.
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This question has been discussed a lot on some sites. Do a search on audioasylum tube forum or the bottlehead forum.
Clearly, Tube life has a lot to do with the voltage running through the filament(or cathode or whatever you want to call it.) Checking that out and adding a Resister if needed)can add life if you are into DIY. The general rule is use the lowest filament voltage to properly operate.
I remember reading a post by Paul Joppa, one of the main brains at the Bottlehead site and he used the figure one hour which he said was accurate to within a factor of 10(for reasons he did not explain). So he apparently thinks the time frame is from 6 minutes (1/10th of and hour) and 10 hours. I guess that means turn it off overnight and when you leave for the day. I'm not smart enough to argue with PJ.
Some folks worry about fire and AC spikes too if you leave it on all the time.
Enjoy the tubes!
The characteristics of the particular tube matters. In my experience, for example, 6922s wear out when left on continuously but good NOS 12AX7 tubes don't. Unless you can find a specific reference to you tube type I would suggest turning it off based on a 6+ hour cyle time. You should also listen carefully to the warm up characteristics. Some tube gear stabilizes in an hour or two and some sounds terrible for the first 4-6 hours. This might not matter if during the day you are casually listening and doing your serious litening in the evening since the tube would have had the whole day to warm up.
My suggestion is to get some Pearl tube coolers and fit them. Not only have I found that they make the tubes last much longer (I don't know how long, all my small signal tubes are still ok after several years use - I switch my amps off at night), but they seem to help microphonics as well.
Only downside is that you see less of that lovely tube glow, but there is no free lunch!
If I recall rightly our Albert Porter, leaving his Aesthetics front end on all the time, has reported here, that the deterioration of his NOS tubes within the course of one year was minimal. I can come up with the same message, regarding my Jadis PrePre stocked with Tele ECC83s, which still measured good after three years. The same holds good with the stock powersupply tubes. By the way, I've used the Ensemble socks in the Teles. If that did any good, I cannot say.