The best insoles for flat feet are quite durable not only because they are made out of firm materials that can make them quite rigid but they are designed to stand the wear and tear of running as well as other types of exercises and a lot of these insoles have material that is breathable for your feet as well the ability to absorb shocks from the impact of running. The firmer the material that is in the insoles can mean that the insoles can last for a long time. Depending on how long and how often you wear them really depends on how long they will last. The current pair I have in my running shoes have lasted a little over a year with no issues.
Because there are so many insoles to choose from it can be easy to forget what you should be looking for so I will remind you of the three basic things you should be thinking about while you are shopping: if the insoles feel comfortable on your feet, are the insoles able to relieve your pain that you could feel in your knees and ankles and help you stop any more injuries as well as to help with you flat feet; will the insoles make your running performance better and more often.