Why Power Tubes Wear Out
Tubes wear out mainly from the heat it generates. A tube dissipates power when it is operating, and this generates heat. Power tubes dissipate a lot of power, which means it generates a lot of heat.
The heat causes metal to outgas, introducing charged gas molecules (ions) inside the vacuum tube. Outgassing happens gradually over time, and Vacuum tubes have getter material that absorbs the ion molecules.
The getter eventually wears out, and tubes become gassy. By the time this happens, the tube will not operate properly. Even before this state, heavily used power tubes will have some gas internally, which hinders their performance.
How Long Before Power Tubes Are Worn
How long it takes to get to this state depends on several things.
First, tubes wear with more use. If you use the amp an hour a day, the tubes will last longer than if you were to use the amp 10 hours a day.
On most tube amps, louder output means more heat dissipation. So higher output will wear out the tubes faster. On amps with class A operation is an exception to this. On class A amps, tubes dissipate the maximum power always. So just having it on wears out the power tubes at max rate regardless of volume level on class A amps. Single-ended amps are class A.
Additionally, certain tube types just run hotter and wear out faster. For example, EL84 doesn't output as much power as 6L6GC but runs much hotter due to its smaller size. EL84 tends to wear out quicker than a 6L6GC for this reason.
Replacing Power Tubes
Because of all this variability, it isn't easy to estimate the life expectancy of power tubes. The general guide is to replace every 500-750 hours of use. Using your amp 10 hours a week (2 hours a day, five days a week) translates to just about a year to 1.5 years.
If you are a gigging musician or if you want your audiophile amp to operate at near-top performance, you may want to shorten this replacement time. This way, you avoid using power tubes closer to the end of the life cycle. Replacing the power tubes every six months is better on a heavily used amp.