how long should tubes last

I'd think of another question why simpliest vacuum triode such as 300b worth a lot more than penthode KT88?

Tube life will vary with the type of circuit the tube is used. Some amps drive the tubes harder than others.

As far as out put tubes goes the simple triodes seem to have mythical lifespansa which I have never heard anyone define. For Pentode and Quatrode outputs the hours are defined and relatively short. If driven hard (as was already mentioned) a typical lifespan maybe 2-3,000 Hrs. Small signal tubes can last 10,000 or more if built to do that. In truth I don't measure hours on my gear but after a couple of years outputs in my PP style amps seem to need replacing . The inputs seem fine.