Alternators are expected to last from 7 to 10 years or up to 100,000 miles.
However I have a 1992 Chevy Silverado truck that has the original alternator on it and the truck has 300,000 miles on it.
The alternator has never needed replaced during that time. But that doesn't mean all alternators will last that long and some may need replaced sooner.
If I plan to go on a long road trip however though and the alternator on my vehicle is 7 years or older then it gets replaced with a new one just to prevent a breakdown on the road trip.
If your alternator is 7 years old then it's a good idea to at least get the alternator tested to see if it may be failing or if it's okay.