how long should you use tubes in an amp?

For sure it would be measured in hours. I burn thru a set of tubes on average between 2000 and 3000 hours which takes place in about one year. My stuff operates at least 8 hours a day or more.
There are a lot of varibles in power tube life, not the least of which is the design and quality of manufacture of the tube.

Most importantly is how hard they are driven by the amp's design as well as user selection of bias points. For example Sonic Frontiers in an old SF 80 (my first tube amp) recommended a bias current of 50ma on some KT99A's and said the tubes would last about 1500 hours, but that you could lower the bias point to 40ma and double the life of the tube to 3000 hours. Some amps are very conservatively designed and tubes can last a very long time. For example the SF amp has four power tubes producing 80 watts, where as another amp I have has 8 similar power tubes producing 80 watts and another with 4 tubes producing 40 watts. Both run relatively cool.

Another thing to consider is the mode of operation when you can make a selection between triode and pentode/ultralinear. Triode will run much cooler and that should also extend tube life.

BTW, Lloydc, one of the best ways to see if your tubes need to be replaced is to keep a spare set on hand and if you think your amp may be reaching the point were new tubes would help is to simply replace them. If you don't hear a change to a cleaner more dynamic sound, put the old ones back in and relax for another year. :-)

Thanks, all. Sensible advice (I assume that Rothmanbrad was more or less joking, but that his point was "pick an answer in the ballpark and don't obsess about it" - entirely reasonable, imho - then again, the Baron uses 12 output tubes, so replacement with nos is not exactly a trivial expense - so far, I prefer Siemans EL34's). I forgot that Mr. Smith addressed the issue in the manual; indeed, who would know better? I rebiased (easy on a Baron) and will forget about it for another year. I do keep spare sets, but they all sound a little different.

The tubes will let you know when they need replacing!

Newbee is right as usual. Tube life quite generally depends on the quality of the tube and the design parameters of the component the tube is used in. The trick with a spare set of new tubes to test the ones you are using is foolproof and reliable. I could kick myself for not having thought of that before...and I thought, that I was a seasoned audionut.....Thanks Newbee.

Lloydc, I have found the Siemens EL34 to be the best in my Music Reference RM9 Special, whihc fortunately is suppose to get 10,000 hours on the output tubes, maybe longer on my easy to drive Merlin VSMs.