I've got about 15,000 miles on my car w/o any issues. With as normal as any Mustang enthusiast would drive a 400hp+ car, my clutch still appears to be doing fine.
In past Mustangs, this is the first brand Mustang I've owned for longer than 6-months, but in my other 9-Mustangs over the last 18-years, I've had to put clutches in about half of them, but not because of failure, but because I didn't like the way the clutch felt.
Also sorry to report that the other posters are correct about the clutch life. The clutch is a wear item, and while they shouldn't wear out in under 12,000 miles, it totally up to the driver to make the clutch last. You could get 1000-miles or over 100,000 miles. Think of it this way, if I were to gingerly drive up to 140mph each day, not stressing the car, and then slam on the brakes until I stopped, how long should I expect the brakes to last? Certainly not very long. Back to your clutch, as long as your being honest to us about your driving style, you should get over 50,000 miles on your clutch.
From my recollection, all the people on here that have had clutch failures so far were from the bolts loosening up, and the issue you've described, with the engagement getting worse, goes right along with the bolts loosening up. My suggestion, find a different Ford dealer in your area.
If my 2011 clutch fails, I'll provide all the details and pics to go with it, because I will probably replace the clutch myself, not Ford.
2015 Mustang GT PP 401A Black | Roush Supercharger