How often do serpentine belts need to be replaced?

well they have been on the car for 6 years!even though you only drive an average of 5500 miles a year,they have been in extreme heat.they belt probably has cracks all around it,and maybe frayed.i would say yes that's probably true.you sure don't want it to break.it runs the water pump,a/c and alternator,and will strand you if it breaks!