It depends heavily on use. In a closet, the tubes will last forever, of course. For practice in a bedroom a couple of times a week at modest volumes, you’ll probably get five to ten years out of them. If you practice twice a week for a couple of hours at full volume and play two gigs a weekend, count on one to two years out of a set of output tubes. Note that this assumes that you got good ones to begin with and that you had them properly biased when they were put in.
Tubes wear out by sheer hours of being turned on, by how hard they’re worked, how hot they get from just the heat in the box, by the number of times they’re turned on and off (thermal shock). Notice that being played at maximum warp into a dummy load (or power brake, or attenuator, etc.) counts as being played hard, and that because you can’t hear all the sound, you may not think that you’re working them hard.
Your ears tell you when to replace them. When they no longer sound quite as punch and sweet as they used to, start thinking about changing them.
I have a somewhat more extreme approach, myself. The best time to get new tubes is when you DON’T need them. You get the chance to find the best tubes at the best price without time pressure. I prefer to keep a whole spare set ready. That way, a sudden burn out will not cripple the amp, and I can readily tell when one of them is just not sounding right by subbing in a replacement that I already have. Be prepared!