There are varying mileage or monthly change intervals depending on the engine size in the Ford Focus, but the mileage interval is either 100,000 or 125,000 miles. Those with 100,000-mile intervals have a 96-month time interval, and there is a 120-month time interval for the 125,000-mile timing belt.
A timing belt wears out from consistent use of the engine, which determines the mileage rating for change intervals. An interval referring to time is given so that older belts are changed to avoid problems due to degradation. Older cars used a timing chain, and wear or age could be determined by the noise produced when in use. Newer engines typically use a quieter, toothed belt that gives little or no indication of a problem, which makes mileage or time change intervals important.