Typically, alternators are designed to last between 80,000 - 150,000 miles. This means that you'll have to replace the alternator once every seven years or so. However, make sure that you aren't wearing out your alternator prematurely. An overtightened serpentine belt, jump-starting the car in the wrong way, being subjected to extreme temperatures and fluid leaks can all cause an alternator to wear out prematurely.