My TCH also has Bridgestone tires. Mine measure 8/32 tread on all four tires at 17K miles. New they were 10/32. My guess is I should get around 50K miles out of them by the time they reach 4/32 (I normally replace my tires at 4/32)

Check your alignment. I had the dealer check mine, and both the front and rear toe was off. Bad alignment can seriously shorten your tires life.

For what its worth, I run 42psi front and 40psi rear with my tires as I find the traction and steering much improved.

Check out the Bridgestone Potenza G 009. A traction, A temperature, excellent handling in dry, wet and snow. Rated excellent for quiet and treadwear. They are $74 at tirerack.com