Knives, like cars, motorcycles, tools, are all depending on how much they are used, and how well they are treated. You can kill a car in 50,000 miles, or get 200,000 thousand miles on it. Heck, buy friend Hoppy got almost 300,000 out of a 1986 Dodge Caravan by treating it well, and being anal on the maintenance.
A pocket knife used on a construction site will not last as long as a pocket knife carried by a suit guy who uses it lightly. Every single sharpening wears away a little bit of blade. Every opening or closing is flexing that spring a little. But in general, I think the average pocket knife should last about 20 to 25 years if used much. I used the heck out my Buck 301 stockman for 25 years, and it needed a new main blade and some spa time at Buck. The sheepsfoots blade was pretty worn too. At 25 years, it was on the border of being owen out, as it was my main edc all that time. On the other hand, my dad got about 40 years out of a Case peanut, but he was a suit guy, and really didn't use his knife heavily. If it got heavy or dirty, he had other things stashed around, like the 10 inch bushwhacker and an old butcher knife in the picnic basket.
Carl.