I start thinking about getting new ones at 300 miles. Since I alternate shoes, I'll get new ones and wear the old ones occassionally until I get close to 400. Both pairs are getting up there now. 331 and 294. Looks like I need two pairs. When I got hurt I started back with 2 new pairs. Usually there's more of a difference between the two pairs. My racing/fast trainers have 182 miles on them. I usually replace them by 250 since they aren't as durable.