when it starts to dry out... look at the ribs and look for cracks. I just replaced mine in my 03; 65k on it and it was factory. Really wasn't in that bad of shape either. Anyway, they aren't that expensive, I usually replace them every 40-60k, just depends on when I get around to it.
Just my own two cents, pick up a good year gatorback... perhaps the best belt on the market