I have personally come to the point in my life where I replace my bike tires before they start to look really bad and if I'm buying a used bike I figure in the cost of new tires before taking the first ride. Not saying I'm going to actually replace the tires but I calculate the cost in with the purchase. I have about 2000 miles that I put on a bike I bought used and still have the tires that came with it but I inspect them before every ride and have new tires ready to put on for next year. The tires that came with that bike are premium tires and are going to be replaced with premium tires. On another bike I had a pair of Performance Bike house brand tires with about less than 500 miles when both failed within two days of each other. You get what you pay for.