I have been tubing for more than two years, and I have yet to replace
any tube in my equipment due to it dying on me.
I replace tubes to change the sound (tube rolling) but those tubes that are removed are still fine.
The shortest-lived tubes I have come across are the EL34s in my Dynaco ST-70. 2,000 hours
daw ang buhay. At 20 hours listening a week, that would be a lifespan of roughly two years. But they are still working!
EL34s are very easy to get and are relatively cheap, if you buy new production, not NOS.
Other tubes (preamp, etc) are rated up to 10,000 hours. That would be 10 years at 20 hours a week.