Alternators on vehicles should last from 7 years to even 20 years before needing replaced.
It depends on how often you drive the vehicle too.
I have a truck that has the original alternator on it since it was new 20 years ago but I think I'm gonna replace the alternator soon just to prevent problems.
The most common reason an alternator fails is due to a bad diode or regulator and when that happens it's best to replace the alternator with a new one.