There are numerous factors that determine the life of a tire. Some of these include the speed at which you drive your car, the tire rotations, its average air pressure, the distances driven, how one accelerates and the payload of vehicle. In addition, the load that a vehicle carries also determines how long it should take before getting new ones.
It will totally depend on what brand/type of tires they are, as it can really vary. Some are only rated for 25,000 miles (or less) and others 60,000 to 75,000 miles (or more). So without knowing what kind you have, there's no way to tell.
Look to see if the tread is wearing down - you'll know when it's time for new ones. Even if they are rated for a certain mileage sometimes they have to be replaced before that if they did not wear down evenly because of poor alignment etc.