I think just about everyone has brought up some good information so far. And really the consensus is that it really depends on numerous factors. I'm going to do my best to lay out as many factors as I can think of that might affect a ski's lifetime.
1. Construction (Wood vs. Composite Cores, Metal Layers, Sidewall vs. Cap, etc.)
2. Skier Type (Hard charging 9-5 vs. Nice and Easy 10-4 w/ lodge breaks)
3. Terrain Type (Bumps vs. Powder, Rocks & Ice vs. Colorado Courderoy)
4. Brand (Some brands are notorious for lesser quality products)
5. Ski Condition (Regularly Tuned vs. Left in a Thule for Weeks at a time)
I'm sure there are some other factors, but those are the ones I can think of off the top of my head. I think the last one (Ski Condition) is worth bringing up as it hasn't been mentioned yet, and can be the quickest way to wear out your skis. Think about for instance if you get a chip in the topsheet or base of your ski that makes it all the way to the core material. If you don't get that repaired, you're probably going to end up with water soaking into the core. When this happens, its only a matter of time before your skis lose their liveliness and begin feeling sluggish.
Of course if I were to throw a number at it, I'd say your average frontside groomer ski should last 100-150 days, given it's cared for and it's quality construction. If you're skiing hard, either in the bumps, down steeps, or in the terrain park, expect that number to be a lot lower. On the other hand, if we're talking powder skis that only come out on deep days, then you'll probably be able to ski them until they've been made obsolete thanks to new technologies.
Hope this helps!
Matt w/ Skiessentials.com