How many miles should you take off a shoe before you retire them? To speak in terms of mileage is a little difficult because there are so many variables. For example, what do you weigh, what are your bio-mechanics like, what surfaces do you normally train on, what shoes do you have, etc.
However, there are some indicators that let us know when it is time to trade in an older pair. Look at the wear on the outer soles. Near the heel on the outside (this is where most people strike and wear out the shoe) is the wear approaching or into the white mid-sole? If so, it is getting or past time for a change.
On the forefoot, are the lug patterns disappearing? This is another sign of the need for a change. While these are rather objective measures of determining the time for change, a more subjective one is whether your ankles, knees, or hips are starting to ache and you cannot determine why. Everything else seems the same.
Think about it. Almost by definition we are destructive to running shoes. Take your body weight, multiply it by a factor of 3 or 4, then consider that you strike the ground with each foot 700-800 times per mile. Is it any wonder that the mid-sole starts to lose some of its cushioning?
After dancing around your initial question about mileage I would say that after you have 400 miles on the shoes it is time to critically assess their condition. Some people can get more good miles and some are better served changing more frequently.