Depends on the tire, the roads, debris, your weight, and which constellation was rising when you put them on. Race tires can be worn out in a few hundred miles, while other tires may go thousands.
If they look good, ride 'em. Threads showin? Then it's time to replace 'em. It's absolutely normal to get 2000, 3000, 4000, or more miles out of a set of tires (training, not necessarily race tires).
Check RBR reviews on tires, and see how the ones you're using have faired. While the reviews' subjective ratings might not be worth much, the mileage on a given tire that's often reported can be a valuable metric.