Spare tires are typically rated for between 50 and 70 miles of use. This is because they are built to be smaller for weight, space and cost savings, and as such they don't have the durability and stability found in regular tires.
The strength of a tire, in terms of puncture resistance and wear, comes from its many layers of steel belts and polyester that lie beneath the rubber tread. Spare tires are manufactured with half as many layers as regular tires. Also, since they are smaller, they don't develop as much traction as regular tires and must turn faster in order to keep up with the other tires. This causes a loss in cornering performance and increases wear and strain to the mechanical parts of the car.