Its quite simple really, replacing your shoes is crucial with respect to injury prevention. The weak link in any shoe is the midsole , the foam or polyurethane material that sits between you and the ground which provides cushioning. When this breaks down you are running the risk of limb soreness, sore kness and hips. Do not wait until the outsole wears out . The outsole is stronger than the midsole and in most cases outlasts the midsole . The other sign of shoe breakdown is the shoe loses its torsional strenght , If you grab the heel and the forefoot and twist the shoe in opposite direction , what once was stiff is now flimsy , this is common with someone who is flatfooted or who requires what once was a more supportive shoe to compensate for the biomechanical weakness, assuming you purchased the correct shoe in the first place. 300-500 miles 500-800K is what you can expect from the midsole , @b 20 miles per week , thats 1000 miles a year, so 6 months, if you are rotating your footwear , I would say annual replacement at the very least.