Tube life

Right now my only tube piece is my phono amp which uses 6922s.
What I care about most is noise. Roger Modjeski's Tube Audio Store tests all their tubes and hand selects them. They then sell them in grades.  Their Super Low Noise 6922s were embarrassingly quieter than the stock tubes from ARC. I keep the thing on continuously. The tubes are 6 years old and as far as I can tell going strong. I have a spare set of tubes just in case something happens so I will let the working tubes go as far a they can. If you have a tube unit do check Roger's site. Every tube he sent me is dead quiet. The SLN tubes are more expensive and worth every penny. 

I did a lot of auditioning of both low level tubes and power tubes in preamp, power amp and phono stage, and found that there was no reliable general rule - some brands sounded better in one use and another in a different use.  It is cheap to try some of the tubes so I bought two or three different ones just to see what was what and used the ones I preferred (based on blind listening by myself and another person).

Auditioning power output tubes could get pretty expensive, though....

Tube life in most gear is quite good - and in some applications they seem to last forever without deterioration. In phono stages, for instance, you'd have to be a vinyl nut that played for hours every day to age out the tubes in a decade.

As was stated, though, it does depend on the amp design and how highly 'stressed' a tube may be in a given application - push the voltage and shorten the life.

I once owned an all-tube phono preamp (Icon) and only gradually noticed its sound no longer seemed up to snuff.  New tubes changed that dramatically.  BTW, the mfg. admitted the tubes were driven hard in that application. 

As far as tube changes making little difference, I have to emphatically disagree.  Changing out KT-88s for KT-150s in my Primaluna Integrated some years ago gave terrific power but dramatically sacrificed timbre.  New preamp tubes in that same amp turned the sound so crisp I sold them nearly immediately (failing to let them break in, alas).  More recently, in a Vincent tube integrated amp, I could not get the highs in my rather live room to settle down. New tubes not only retained the high-end and made it more natural, they also dramatically filled out the bass.  None of these sonic changes were even subtle!