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I would contact Rogue Audio and ask them how long you can expect the tubes to last in your preamp. They should be able to give you average life expectancy of the tubes they decided to use in their preamps.
It also depends on how many hours you are using the preamp on a daily, or weekly basis of course.
from the FAQ page on Rogue's website:
How long do the tubes last?
Preamp tubes and the small tubes in the power amps typically last at least 5000 hours before they need to be replaced. The larger power tubes in the power amplifiers depend somewhat on how hard they are used. "Typical" lifetimes are about 3000 hours which equates to about 3 years if used for a few hours every day.
I've pulled small signal tubes out of 70 year old radios / equipment that were still working. really depends on like others said how hard they are run also the quality of the tube. IMO, NOS tubes tend to last much longer then their new counterparts but are usually much more expensive so its a trade off. NOS tend to sound better but not always. New production tubes are getting better every year but also their prices have steadily risen.
12AU7's are very common so you will never have issues finding them new or NOS so don't worry.
Nos Mullards or telefunkens will last as long as you own your equipment. I’ve got some from the 50s and 60s that still test 90%. They must have at least 20 k hours on them. Obviously mine are not nos but I’d feel real comfortable paying good mo eye for them from Brent Jesse or another reputable tube seller.