I weigh 175 lbs. I can usually get 5,000 to 7,000 km on any front tire (usually 200 gram folding), rear tires are a different matter. The best tires I've found are Vredestein Tri-Comps, which will run to about 4500 km.
I replace the tire when it gets a cut in the tread that requires a boot to fix (otherwise the inner tube will stick out, get worn down, and start leaking). Very seldom do tires wear down to the cord here in Vancouver, especially when the roads are wet (most of the time).
Front tires stay on the front, rear tires on the rear; I'm not a believer in "tire rotation." I like putting a new tire on the back instead of a worn front tire. Plus, my front tires specifically front tires, usually 200 grams, the rear tires 230 to 250.
L.