Timing belt replaced on 1998 Ford explorer at what mileage?

It has 160,300 miles on it but the owner doesn't know when the timing belt wad last replaced. How many miles should the timing belt be replaced

Are you sure it has a belt? What engine is in it.

Not a belt but a steel chain. It does not have a recommended time/mileage to change. The time to change it is if something is going wrong with it; usually over 200,000 miles.