There is no exact mileage when a timing belt will break. Some belts fail early due to oil or coolant contamination, poor tension, faulty pulleys, defective parts, or harsh operating conditions. Others last beyond the recommended interval, but that does not make the risk acceptable. Most manufacturers set replacement intervals around 60,000 to 100,000 miles, and some newer vehicles may be closer to 120,000 miles. Once a belt is past its service interval, the chance of failure rises because the rubber and internal cords have aged through thousands of heat cycles. If the mileage is unknown, assume the belt is due until service records prove otherwise. Preventive replacement is cheaper than failure, especially for interference engines with expensive internal components.