They have worn down just like my old shoes if not a little better since I don't hit as hard now. I'd say they last me a good 400-500 miles before I start to notice the tread going.
I always buy them 2 pair at a time online and alternate them during the week when I run so that I have broken in shoes without too many miles for races. Plus then I only pay shipping once.
P.S. today is a rest day. I love rest days! I didn't even set an alarm this morning and when I automatically woke up at 5:15 am I just rolled over and went back to sleep with no guilt :)