See Figures 1, 2, 3, 4, 5 and 6
The CA and VG series engines utilizes a timing belt to drive the camshaft from the crankshaft's turning motion and to maintain proper valve timing. Some manufacturers 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 (manufacturers with interference motors) to prevent the possibility of severe internal engine damage should the belt break.
Although theses engines are not listed as an 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. Nissan does not publish a replacement interval for these motors, but most belt manufacturers recommend intervals anywhere from 45,000 miles (72,500 km) to 90,000 miles (145,000 km). You will have to decide for yourself if the peace of mind offered by a new belt is worth it on higher mileage engines.
Whether or not you decide to replace it, you 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.