I used to get injured around 500-600 miles and would have to replace my shoes, but maintenance exercises, primarily eccentric calf drops for the achilles, since then have allowed me to get at least 1000 miles on every pair of trainer for the past six years. I've been cycling through three identical pairs of trainers since May 2013, during which time I've probably run about 5000-6000 miles. However, the sole is starting to peel away from one and the tread is almost all gone on all of them and I'm getting quite a bit of heel pain, so the end is nigh. One thing that will make your shoes last longer is to put newspaper in them and dry them out in the sun after runs when it is hot.