The 5S-FE and 7A-FE engines utilizes a timing belt to drive the camshaft from the crankshaft's turning motion and to maintain proper valve timing. Some manufacturer's schedule periodic timing belt replacement to assure optimum engine performance, to make sure the motorist is never stranded should the belt break (as the engine will stop instantly) and for some (manufacturer's with interference motors) to prevent the possibility of severe internal engine damage should the belt break.
Although the 5S-FE and 7A-FE engines are not listed as interference motors (it is not listed by the manufacturer as a motor whose valves might contact the pistons if the camshaft was rotated separately from the crankshaft) the first 2 reasons for periodic replacement still apply. Toyota publishes a replacement interval for these motors. The recommend interval is 60,000 miles (96,000 km). Refer to Engine & Engine Overhaul for Removal & Installation.
It would be wise to check it periodically to make sure it has not become damaged or worn. Generally speaking, a severely worn belt may cause engine performance to drop dramatically, but a damaged belt (which could give out suddenly) may not give as much warning. In general, any time the engine timing cover(s) is(are) removed you should inspect the belt for premature parting, severe cracks or missing teeth.