When do you change a Serpentine belt?

You can look at the grooved side and look for a bunch of small cracks , carry one on board or change it out as preventive maintenance , not that expensive or hard to do , Shouldn't be anyways

If you see the cracking like beatupchevy described or when the belt tensioner is near the end of its travel. A good quality belt can last many years and many thousands of miles. Hope that helps! Jim