I changed my Bridgestone Potenza's at around 24.5k when I put my new wheels on, and they still had around 4mm of tread left on the fronts.
I'm running Goodyear Eagle F1 Asymmetric 2's now, and don't expect to get as many miles out of them, but I didn't like the Bridgestones, so changing my wheels gave me a good excuse to change my tyres.