the clutch all depends on how hard its used and if proper shifting is being performed. if you shift like a tool and "grind" gears a lot, you'll see it slip fast.
One of the guys at the dealership i go to said that this girl came in after 100 miles on her brand new lancer, burnt her clutch. like WOW, bad driver???!!
I bought my 06 GT as the 3rd owner at 38,000 miles. it started slipping around 40k. I was pissed, but I assumed that it was beat to shit before I bought it. Replaced it with a new stock clutch a few thousand miles later. im @ 79k miles now and no problems!
You can easily tell if your clutch is slipping. for me, it started slipping in 6th, then progressively started slipping thru 5th, 4th, 3rd, then also in 2nd gear. You push down on the gas pedal and your RPM's just shoot up then go back down. feed it too much gas and it will continue to slip. treating the pedal like theres an egg underneath shouldn't cause too much further damage until you can afford to swap out the old clutch with a new one.
if you continue to let your clutch slip, you will wear out and basically torch your flywheel much faster, and eventually you will NOT be able to move the car at all until all of it gets replaced.