For preamp tubes here's the rule of thumb:
If it aint broke don't fix it...
Ok, now that we got that out, it is a good idea to keep at least one preamp tube handy incase you need one because one decides to fail.
There is no set life expectancy for any tube, though the old stuff tended to last longer by this I mean the vintage tubes back when there was good quality steel, QA and vacuum processes. For some reason, I am thinking the lack of tube use in military and medical applications as well as massive commercial and private use, tube quality has fallen off in production. The once good manufacturers gave up and sold their machinery to up and comers. Unfortunately, being that there is less demand globally because of modernization of archaic technologies, there is less demand. Without the demand, there is not as much reason for the tube manufacturers to make as good of a product as once there was. A lot of the old machinery needs to be retooled and just plain replaced. It is hard to make a diamond first out of crap then also on crap machinery.
The real thing to look at when maintaining your tube amp is how hard you push it. If you are a gain freak and use a high gain amp like most of us here have then chances are you are going to be buying more preamp tubes than someone using something that doesn't abuse the little guys, ie. your uncle's old Bandmaster that he plays traditional folk music on. If you are performing regularly (3-4+ shows a week) and getting paid for it, I would suggest that you replace your current production preamp tubes at least every other month just to be sure that you have no surprizes. Sounds like a lot but if you are performing that often chances are you have some kind of endorsement or at least are making enough from your playing to do it. Microphonics can show up at any time and rigors of a tough schedule can beat up your tubes. Friends of mine that work with larger acts say that they do a full retube once a month whether they need it or not just as preventive maintenance and to ensure that the tone is always the same for every show. For someone that just plays at home you can get away with just replacing them as they go bad. If you are gigging, then just be prepared with spares as necessary. Again as was stated above, your power tubes take the hardest beating. Be prepared to replace them faster than preamp tubes.
