Serpentine belts last between 80,000 to 100,000 miles and sometimes longer.
Although after 10 years I replace my serpentine belts just as preventative maintenance and to prevent it breaking on the road.
I will keep the old belt as a spare just in case something happens or someone else might happen to need the belt on the road.
I also replace the serpentine belt pulleys after 10 years as well to keep from having them seize up and then cause my new serpentine belt to shred.
I never want to wait until the serpentine belt actually breaks because it can leave you stranded and can cause your engine to overheat as well due to loss of coolant.
I hate being broken down on the highway or anywhere else and having to fix things on the road so I replace the belts and hoses before they break after 10 years regardless if they are still okay.
10 years is a long time if you think about it because 10 of those 10 years is 100 years.
Same goes for tires I replace those every 10 years or sooner if I drive more often although I do have 20 year old tires on an old farm truck that only sees driving around the small down and doesn't go down the highway so if those tires blew out it wouldn't be a problem.
But on the highway it would be a problem.