There are far too many variables to produce a single answer. People strike the ground differently (for example, I land on my forefoot and a burn through shoes because there is very little support built into that part of the shoe), they run on different surfaces (roads, grass, trails, etc.), and different shoes have different compositions (some are made to be more durable while others are built to offer more cushion). I usually wear a neutral shoe (no sense in getting a supportive shoe when I don't land on my heel) with a sole that doesn't have much spring and I'm lucky to get 200 miles out of a pair. I recommend getting new shoes when you start seeing any significant wear on the soles or you start to have general soreness in your legs.