The machine was designed to run for two years at rated (full) power for the average home user. What is “average” is undefined, and variations in tube life are significant. Most “hobby” rated tubes seem to be about 2,000-3,000hrs. Industrial units are more like 10,000hrs.
Running at lower power won’t necessarily increase the life of the tube. Tubes also have a shelf life, so not using it won’t necessarily extend its life significantly, but if you’re only an intermittent user, you could certainly expect more than two years. I’m coming up on three.