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.

Try to find out what plate voltage and what current is applied to the output tube. Then you can calculate the plate dissipation in Watts. Then find out what are the manufacturer's suggested max plate voltage, current, and dissipation. If the plate voltage, plate current, and the product of the two (Watts) are not more than 60 to 70% of the respective maxima, then the lifespan of the tube will be at least not reduced by the circuit demands. Within those limitations, you might get an idea of which of the two types you mention will last longer by looking for the manufacturer's estimate of lifespan. Just keep in mind that the numbers given are "on average" and may or may not apply in a given instance.

10-11-12: Marakanetz
I'd think of another question why simpliest vacuum triode such as 300b worth a lot more than penthode KT88?

My guess would be that because the Triodes are not used in guitar amps (and many fewer audio amps) the economies of scale just aren't there

Some circuits drive tubes harder, but the speaker also has a lot to do with tube life. The more difficult the speaker load and the sensitivity of the speaker are key factors. For example, tubes will last much longer driving a 100db 8 ohm speakers than an electrostatic speaker with an impedance ranging from 50 ohms to .5 ohms with a sensitivity of 83db.