I've got Specialized Armadillo Elites and I get about a year out of a set (2,500-3,000 miles) but I usually change tires before they get to the frequent flats stage and pass the used ones on to the local bike co-op while there are still a few miles left in them. The Armadillos aren't a fast tire by any means, but they are about as flat resistant as anything I have tried.

Two flats over the summer is hardly a bad average.