When you have experienced a timing belt break, and have counted the cost, you will not ask such questions. But the above suggestions are quite right, it's time and mileage together that bring a belt to the end of it's life, and 5 years or about 100,000 miles whichever comes first, is a reasonable yardstick. However some engines [Ford 1753cc diesel] have 2 belts, only the camshaft drive belt is critical. The injector pump drive belt can be neglected because when it breaks the engine just stops, doing no mechanical damage, and only the camshaft drive belt breaking will do mechanical damage..