The typical billet types made of 6061 AL wear really quickly, the 7075 types last much longer...almost as long as steel. The chrome ones slip unless you bead blast the vee'd surfaces first, some people have no problems if the belts are installed overtightened. Most people have no problems whatsoever since they are mildly street driven and never notice any slippage, it's mostly a racing high load/high rpm problem. The quality chrome ones seem to last forever.
Depends how hard and how much you drive the car.
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