My general rule of thumb is that the rear tire will wear out about three-times faster than the front--so that I replace the rear tire about three times as often as the front. I don't bother with rotating the tires--once the rear is worn out, it gets replaced with a new one, and the front stays where it is. When do I know when it's time to replace the tire? When the rear tire has worn down flat and starts to give me that "hmm...I should replace it" feeling. As for the front, I inspect it regularly for cracks/gouges/punctures and wear. If there's too many of any of the aforementioned blemishes (or gives me that "hmm...I should replace it" feeling), then it gets replaced.