Rule of thumb is every 6 months or 500 miles, whichever comes first. Heavier runners should replace their shoes close to the 350 mile mark while lighter runners can stretch shoe mileage closer to the 550 mile range. If you run around 25 miles per week, you should replace you shoes every three to four months. Remember that the midsole wears out way before the outsole and runners who wear out their shoes faster than this should be evaluated for bio-mechanical stress problems.