depends almost entirely on the type of skiing you do, and how hard you ski. if yer just cruising around groomers, skis would last years and years. if you do alot of rails, skis are gonna get beat up alot faster. if you ski at a super rocky mountain (which i do) your skis will sustain more damage faster. if yer going out and charging and hitting big airs and whatnot, your skis will lose their pop faster and evntually probably fall apart.
i guess the type of ski would factor in also- type of material, how well its put together.
i pretty much ride my skis into the ground- ie busted edges, wobbly bindings, delam, etc. i can tell pretty easily when my skis are "skied out."