I got 18000 miles out of the front G709. It was nearly wore down to the wear bars at 18000 miles and I could have gone another 2000 miles if I only rode on straight roads, but the tire slid a bit for the first time on a curve on a wet road, so I replaced it with another G709.
I got around 11000 miles out of the rear Bridgstone before I got a nail in it and replaced it with a Avon. It was nearly wore out at 11000 miles though. I'm going back to the Bridgestone rear tire when this Avon wears out....the rear Avon's work just as well as the rear Bridgestone does and seems to wear better, but are too noisy in the curves.
Depends on your comfort level. Some folks change theirs when the wear bars get close to the tread level, and others change theirs when the third layer of cord starts to show. 10 to 14K seems to be ballpark.