It really depends on your weight, the shoe you wear, how hard you land and on which part of the foot, what kind of surface you run on, and how much cushion you need. There are a lot of variables.
I go by feel. I can usually get at least 500 miles out of mine but I'm light and land pretty softly under my body (I don't plant the heal). Once I am over 500 I pay attention to how responsive the shoes are and how they feel and usually I can feel them go when they do.