Belts are supposed to be changed every 15,000 miles but mine must have been done already as they looked like new. To be honest Pedro we hardly ever come across a belt that has just snapped on it's own. It's usually because a tensioner or roller has seized , our mechanic and boss can tell if they need changing just by feeling the tension and visually inspecting them. We don't don't just change them because of time period or mileage .