If you notice any starting problems in your vehicle, your alternator may be giving up. So, if you are wondering how long do alternators last? The Answer is as simple as Most auto mechanics will give you a ballpark of between 40,000 and 100,000 miles. It actually all depends on the type of vehicle you're driving. The car may drive for a while with a bad alternator but understand that the alternator is more responsible for charging the battery and keeping all electrical components in your vehicle operational.