The length of time and mileage a car alternator should last depends on the specific vehicle, driving conditions and electrical draw. The average lifespan of a car alternator is 4 to 5 years or approximately 100,000 miles.
A car alternator draws power from and returns power to the car battery when the vehicle is running. Recycling power allows the battery to maintain a charge rather than draining completely as the vehicle runs.
The lifespan of an alternator diminishes more quickly in temperatures of more than 80 degrees Fahrenheit and when the vehicle has a high electrical draw from a malfunctioning element. Installing a battery that is too small or doesn't yield enough voltage also shortens the life of an alternator.