Tube Life: 6SN7, 6DJ8, 12AT7 et al.

We've seen 6SN7s go 50,000 hours if they are NOS varieties. Our experience has been that of the modern 6SN7s (Chinese and Russian), that the Chinese tubes usually last longer, unless you have obtained some NOS Russian types made before about 1987.

The tube life depends heavily on the circuit in which they are used. So I would not rely exclusively on the nature of the tubes themselves- that would be over-generalization.

Anacrusis, IME tubes degrade gradually along a fairly linear curve. So much depends on the circuit and the individual tube though that beyond that any generalizations can be wildly inaccurate. IOW, if you are looking at a particular tube product, see if you can ask the owners how well it holds up. There are forums where this can be fairly easy, for example with our stuff you can inquire here or at http://www.audioasylum.com/forums/otl/bbs.html

Independent forums are likely to be less biased.

Usually you will get more accurate data this way, and its an easier tack then the one you are taking, IMO.

Heat shortens the life of any tube. So cooling is important, and in the case of power tubes, especially if they are in a class AB amplifier, the more power you ask of them, the hotter they get. If you don't deal with the heat effectively (some of this may be handled by the design of the amp), the life will be shortened.