I rotated my fronts to back at 20,000 miles. At 40,000 I was going to do it again, but I managed to pick up a chunk of junk from a wreck that I drove past, in one of my front tires. Since the tire wasn't repairable, I had the spare tire put on the front along with a new tire. I should get another 20k out of the back tires, before I have to replace them.
I figure, under moderate BOMBing and sane driving, most of you should get about 60,000 out of the factory Michelins
Doc