Tubes start to wear out from the moment they are installed. The frequency with which you replace them depends on a number of factors, with usage at the top of the list. Running the amp at top volumes will also shorten tube life. Many artists who regularly use their amps replace the power tubes once a year and have the amp biased at that time. Preamp tubes need only to be replaced when they fail or go microphonic. Power valves/tubes should be replaced prior to failure.