Unfortunately, there isn’t a one-size-fits-all answer to the question of when running shoes have passed their prime and can no longer provide optimal protection against injuries. While I was looking into this subject, I came across a surprising bit of information that I think is important to know when you shop for shoes. In an explanation of how to choose the right running shoes, the American College of Sports Medicine (ACSM) notes that 85 percent of people wear shoes that are too small. You’ll get the most safety for your feet and joints if you buy the right size to start with. The ACSM advises buying these types of shoes from a store that specializes in them. Ideally, the store should have a treadmill for you to try out shoes you’re considering and sales people trained to watch you run in order to help you select the best model for your stride and foot type.
Here’s a brief summary of what the ACSM suggests evaluating when you’re buying running shoes: make sure that you have plenty of room at the widest part of your feet and that the shoes aren’t too tight but not so loose that your feet slide around in them. Your heel should fit snugly into the rear of the shoe.
Once you start wearing the new shoes, the ACSM advises that it can take between 60-70 miles, to break them in. Eventually, however, you’ll have to replace them. According to the ACSM, the rule of thumb is between 400 and 500 miles of running; so you'll have to keep track. That’s a more objective way of calculating when to replace shoes than an arbitrary time limit on their lifespan or visible wear and tear on the tread, midsole and outer soles, which may not show up for many more miles.
Surprisingly, with so many people running and so many injuries from improper footwear, there have been no clinical trials to give us scientific information on how long you can continue to use a pair of running shoes without risking damage. (An article on this subject in the New York Times on February 18, 2013, reported that a Danish researcher is looking for funding for a 15-month study with 600 runners to learn how long you can use shoes without developing physical trauma related to such issues as support and cushioning.)
As a rule, your running shoes will last longer if you run on soft surfaces rather than concrete. Other factors to consider are your weight, your running style and how cushioned or lightweight your shoes are. Certainly, don’t let your shoes fall apart. Bear in mind that worn shoes can worsen an existing foot problem and put you at increased risk of injury.
Andrew Weil, M.D.