I usually inspect the serpentine belt every 6 months. I'm looking for any signs of wear (stress cracks, fraying along belt edge, etc.) that would indicate to me that the belt should be replaced. If & when I change the belt, I'll probably change out the belt tensioner then, too. Even though my car is approaching 13 years old, it only has 24k on the ODO. And both these factors, combined with my driving habits, seemed to have kept the belt in decent shape.