My 700x25 Conti Grand Prix 4 Seasons started out on the front, and is now on the rear, and has over 6,500 miles with no flats. *knocks on fake wood desk*
The other GP4S that I used originally on the back lasted far less than that (3500-4K miles maybe), had a flat on the road once, then spontaneously ruptured with the bike in the work stand, at which point I retired it.