We had a set of Borbett (sp?) that cracked at the spokes after many hours of use. Sift around here, you may find a topic started by me on it. One was badly cracked, then soon after we found several more with smaller cracks. Out of 12 wheels I think 5 we junked.

Inspection is key. Clean them so you can see the surface, avoid dark colored finishes, they hid cracks too much.

I think there are too many different types of wheels to be able to put a finite lifespan on them. Some are tougher than others.