What wears out a timing belt are age, external chemical impact and mechanical wear. Where mechanical wear can be measured by numbers of revolutions of the engine. It is impossible to accurately calculate the exact time when a belt must be replaced. The manufacturer's recommendations are based on figures that allow them to be certain that the belt will not break before that given point. The timing belt on the car that belongs to a young lead-foot showing off his driving skills, would not last as long as the one on his Granddad's car that potters to and from the Bowling Club. Nevertheless, both should be changed within the manufacturer specified intervalls to ensure trouble free operation of the engine.