In my experience of tearing down more than 30 of them - some with very high mileage - I have never found an OEM belt broken.
These belts were the first of their type. (Yes, the fibergalss cord reinforced neoprene belt for OHC timing was invented by Pontiac.) They were very over-designed. They don't stretch like a metal chain; they don't walk off the sprockets; they don't jump teeth on the sprockets and they don't snap for no reason.
I did hear about one jumping a tooth on the sprocket when being driven in heavy snow with a loose timing cover.
