Most timing belts on older vehicles need to be inspected first.Then according to manufacturers recommendations/scheduleing (these parts have been tested,and are pretty accurate) ,it may be between 60- 90,000 thousand miles. Check odometer reading if it is still working, if you hear continous chirping or squealing noises, have "ALL"the belts checked, its always better to be safe, because if a belt such as the timing belt breaks, it can cause engine damage, and you will be stranded.. If you are one of the lucky "third" owners of this vehicle, ask previous owner about repair history if possible, and get references, and shops with knowledgable techs , understand time/labor charges , ask if parts are in stock/ availablity. This belt must be correctly replace it makes the difference between being stranded and driveing worry free, also your timing bellt may run over the water pump pulley, have water pump inspected its probably right there by the timing belt. Now there are some vehicles that can "fudge" this and wait until over a hundred thousand miles, but they must use caution, this could happen if a vehicle say was owned by a "sunday only driver", these belts are rubber and do dry rot over time. Now if after a through check by a repair person, they find that there are some seal leaks, the parts are not that expensive, in most cases, and the tech is right there in the area, have the seal replaced.