I've spent $5k on a clutch-repair. Although $2000 of that was for the CenterForce clutch with could handle 800lb*ft torque. On the track, I can go through 10-15 shifts a minute for 3000-4000 shifts in a single day with about 100-miles of driving. There are two variables that determine a clutch's life: torque-rating and usage-pattern. For the most part, factory clutches can handle factory engine-output just fine and will never slip under normal usage regardless of the how hard you push the throttle.
However the 2nd variable of shifting-technique makes a HUGE difference in whether you get 30k or 300k from a clutch. The most important stage that wears out a clutch is when you release it after a gear shift. IF you have the engine-speed matched to the road-speed of the wheels (through the gearbox), then there is no difference in rotational speed of the pressure-plate versus the clutch-disc and it will last pretty much forever. The way to do this is to rev up the engine to the final RPM that it will be at after the shift BEFORE you release the clutch. That is, if you've shifted from 2nd to 3rd @ 45mph and the engine will be at 3000rpm after you let out the clutch, you want to use your right foot to bring the RPMs up to 3000rpms BEFORE you release the clutch pedal. So the shift-technique is as follows:
1. accelerate up to shift-speed, say 45mph in 2nd gear, engine's around 4500rpms
2. let off gas to maintain steady 45mph speed, this ensures no slippage of clutch when you depress pedal
3. step in clutch pedal, do not let up on gas-pedal, if you did #2 correctly, engine stays at 4500rpms
4. shift gear-lever to next gear, 3rd
5. change engine-speed to final-RPM of next gear, in this case, lower engine to 3000rpm
6. let out clutch, if you did #5 correctly, engine-speed does not change
You'll need to calculate the actual RPMs used in the above examples with your particular gearbox-ratios. Steps #3 and #6 is what determines your clutch's life. If done so that RPM-matching is perfect, the clutch will last over 300k easily. In fact, if RPM-matching is done that precisely, you can skip the clutch-pedal completely and just pull the lever out of gear, adjust RPMs, and shift it into the next gear. No clutch-pedal needed (I do about haft my shifts this way) and again, it will last pretty much forever. Also note that in step #5, if you are downshifting, you actually raise the engine speed when going to the next gear.
In both cases of upshifting or downshifting, you actually never let off the gas completely. If you did, you'd be using the clutch to pull up the engine from idle to the final-RPM, which causes most of the wear. With proper RPM-matching before letting out the clutch, you can keep the speed-differential between the pressure and clutch-plates minimized and prolong its life. You can tell when you're doing this correctly by noticing your passenger's head-bob. If their head doesn't bob forwards and back when you shift, you're going it right and your clutch will last forever. Good practice for shifting-technique to prolong clutch-life is to drive an older car without synchros in the gearbox where you have to do double de-clutching.