Unfortunately, Ford's recommended interval for the 1989 Ranger 2.3L timing belt isn't consistently documented across all sources. However, given the age and potential wear and tear, you should not push beyond 60,000 miles (96,000 km), or 6 years, whichever comes first. Many mechanics would advise replacing it even sooner, especially given the age of the vehicle. Ignoring this could lead to a very expensive engine rebuild or replacement.
Before undertaking any work yourself, consult your owner's manual if you have it, or a reputable repair shop for the most accurate information specific to your vehicle's history and usage.