Davemitchell is correct for the most part. There are some preamps that have a reputation for being hard on tubes, but most should be good for several thousand hours at least. I would not leave a tube pre on 24/7, but if you had it one 10 hrs/day, you should get 1-2 years on one set of tubes. There are many tube pres out there that use 2-4 small signal tubes so buying 1 or 2 tubes/year is no big deal (and there is no biasing involved).
