I agree with Phlakaton. No need to replace purely on mileage. The amount of wear and weathering are all you need to consider. If you garage your bike, and avoid hard stops and starts, your tires will last a good long while.
Side note, your tire profile may be really abnormal though, if you've put a lot of miles on it on straightaways. Make sure to check the tread depth in several places. If has worn square, you'll be thrown off by how rounded your new tires feel.