How often do the tubes need to be replaced?

Tubes do wear out over time. There is a heating element inside that causes a metal cathode to emit electrons that move at high velocity to a metal plate that dissipates even more heat. The vacuum could eventually leak and air molecules inside will collide with electrons enough to make the process no longer work properly. Plus, magical audio spirits in the tubes will whisk away somewhere over time to pursue their eternal destinies. In any case, the tube doesn't last forever if you know what I mean.

This depends on how much time you spend listening to the amp. Remember to always turn it off when not in use, and you'll save lifespan of the tubes. I have had some amplifiers last for many years with the same tubes. And I've accidentally left the amp on overnight quite a few times!

Changing tubes is not like an oil change for your car. You shouldn't feel like, oh wow, it's been a few years, I probably have old tubes that need to be replaced. Keep those guys going as long as you are getting good sound out of them! You'll know if you start hearing noise and hum, lower quality sound, poor volume level or frequency response, the tube no longer heats up and shows the glowing filament, or more obviously if the tube burns out with a sizzle and just doesn't work at all anymore. If the silver "getter" of the tube turns white, the vacuum is lost and this is evidence that air is present and the tube needs to be replaced.

Most of my amps use easy-to-find tubes, not obscure tube models that require you to trade precious gems to faraway merchants. I can point you in a few directions if you need a recommendation on a tube vendor. I usually recommend a reputable dealer, not just anyone selling old tubes on an online auction site.