A tire's lifespan is anywhere between 30,000 to 60,000 miles. This depends on the type of tires you're using, road conditions, your driving habits, and wheel alignment. Most tires have a wear indicator bar at the bottom of the tread grooves that sits laterally to the tire. If the bars are flush with the adjacent ribs, then it's time to replace it. You can also check with a tire-tread gauge which costs around $10. Insert the gauge in the tread grooves, and it will take a reading. As long as it reads above 2/32 inches or 1.6 millimeters, then it's still safe to use.