How do you know when to replace tubes

I didn't count the hours on my 120s but it was about 2 and a half years of
use...even 5 hours a day (I have my rig on more than that usually) is over 5000
hours, and they still held their bias and sounded fine until that last couple of
weeks.

I say use your ears, and have backup tubes around from your god damn tube hoarding fetish...oops...got carried away there...there is NO way anybody else can suggest when to change the tubes on an amp they're not personally listening to so stick the backup tubes in as a reference when you suspect aging... I got about 8000 hours from a set of KT120s with great performance until I fired them (laid them off?) for getting soft, and until my KT150s arrive I'm enjoying the hell out of my backup 88s and 6550s. Note that all of this depends on how hard your amp drives these things, and power tube explosions aren't any more likely than SS amp failure...it happens, but rarely in well designed amps. I recently replaced my 12AX7s after rolling my various tubes around and found that my previous ones were also a bit dull...new set of Gold Lions put that right. This is the fun part of tube amps...love my Flashing Mullards...great band name...

I had the same tubes in my '60 Fender Deluxe for over three decades.

They became more affordable and now cost about the same as 120s on a weight basis (it might take 5 120s to equal 4 150s), they could last a ling time, and they're larger than any other tube that will fit in my amp (!)...I was listening to a great live jazz CD (Fred Hersch) tonight and the 150s sort of woke up and now sound even better than yesterday...they sound large.

LONG time...not "ling" time...my retired 120s were about 100 bucks less than the 150s I just bought (they're still available by the way), and when you adjust for inflation, "present value" of money, Russian raw goods indices, jo mama, gross weight, groceries, Albert Grossman, the savings on heating your hut, the cost of cooling your hut (!), and labor used in wiping fingerprints from the glass, it's not too bad a deal...

Once again, the risk of transformer failure from wildly careless drunk and clearly insane risk taking tube rollers raises its ugly head. I did an informal study of this issue a couple of years ago (this study is ongoing by the way) and determined...ahem...that NO TRANSFORMER FILAMENT HEATING RELATED FAILURE HAS EVER OCCURRED (or has been reported anywhere I've seen) from using 120s or now 150s (although it's early for these...be warned!) EVER in the history of the entire audio world. None. Zero. Also, Jolida techs have approved 150s (and previously 120s) for use in the 502p, I use them now (somehow the amp is surviving, although it could BLOW UP any second) after replacing 120s that got old, and, again, no issues for me or anybody else I've ever read about anywhere...in spite of warnings all over the place. It's possible it's happened, but data on those happenings is nil, nada, none, zilch...still I'd check with yo' mama before going outside, you might get wet.