How often do you change tubes in your tube preamps?

Are you talking mic preamps and recording gear, or guitar preamps?
Tubes in recording gear... it all depends on how much you use them. A good rule of thumb is , with heavy use as in 40 hours per week year round... maybe 5 years of use then chances are things will start to act up.
 
And in guitar/bass tube amps, you practically only change them if you get bored with their sound. Preamp tubes do not have high voltages going through them, so they have very little wear to them. You can easily use the same tubes for 20 years without noticing anything. Usually the tubes break physically, from being knocked during transport.
 
Yep, I was talking about mic preamps. Well I'm glad to hear its something that I won't have to buy on a regular basis, thanks!