It depends on the ski:
Wood/metal sandwich construction race/speed/fast freeskiing skis - 10 years+. I have a pair of first gen stockli stormriders from '00, I expect them to last another 5 to 10 years. DH race skis last forever. *These skis don't become outdated.* Maybe 100 to 200+ days total.
Wood/metal sandwich/torsion box freeskiing skis (head, elan, stockli, nordica, etc) - structurally about 100-200 days. But depending on level of abuse...damage like core shots, severe edge damage, cracked sidewalls, etc, life would make their life shorter. I typically replace my primary freesking ski anyway after 2 years, because something new and better is out, or my preferences have changed.
Wood/metal (torsion box) cap skis (volkl) - about the same as above, though I would guess slightly less. Volkl caps feel like crap when new, so I guess it doesn't mater.
Wood core/sidewall construction skis - structurally about 50-100 days. Several people I ski with are on public enemies, they say about 70 days, and they fall down dead abruptly. But it's a good, hard 70 days.
Foam/metal caps (salomon/rossi higher end, etc) - 20-30 days before they lose thier pop.
Foam/Fiberglass anything - about 3 runs, or 20-30 minutes of skiing. (kidding, but they are by far the least durable)
My primary powder ski this year is an atomic powder plus, I think mine are from the mid 90's. I bought them drilled once, skied once. Wood core, metal laminate, very beefy and durable, great for hitting rocks and skiing tight trees.