Now and then, somebody asks me, "When do you switch out an old tire?"
When the part that touches the road isn't black any more, that's my indicator. Actually, I noticed a few little red blotches a couple weekends ago when I cleaned up the drivetrain, and I've been riding on good luck since.
This tire gave me fantastic service - between May and today, it rolled over 3482 miles - with zero flats! Sah-WEEET! I'm totally sold on Vittoria Randonneurs! (A slightly wider, and significantly better-treaded tire went on, for the winter months. That's good, because there's 2 or 3 inches of snow on the ground out there.)