You often see people looking at the bottom of a running shoe to see if it needs replacing, for example by seeing whether it has much “tread” left. This is not the right test of whether a shoe is finished: the main determinant of the longevity of a shoe is not the extent of wear to the outer sole, it is the compression of the mid-sole, which is the spongy layer between the outer sole and your feet.
Most running shoes today have an EVA mid-sole. EVA is light and absorbs shock well, but it gradually compacts as it is used, which reduces its shock absorbency and gradually distorts the shoe. As a result of the compression of the mid-sole, most running shoes have an average life expectancy of about 300-600 miles.
Very heavy or uneven runners might wear out part of the outer sole before the mid-sole is too compressed, but this is unlikely.
The actual life of your shoes depends on your weight and your running style. You can see whether your shoes are past their best by looking at the compression lines along the side of the shoe, and seeing whether the mid-sole can be compressed with pressure from your thumb.
If you can no longer compress the mid-sole, then it is time to replace the shoes. If you begin to get any kind of ache or pain in your ankle or knee, check that your running shoes don’t need replacing.*