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.
12AU7's are very common so you will never have issues finding them new or NOS so don't worry.