I have an 07 HHR with 45k. About what milage would I get the rear drums replaced?

 

Brakes aren't really a mileage issue. It all depends on how you drive(city vs highway, how hard you are on the brakes, etc). You likely won't have to service the rears until quite a few miles after the fronts need new pads. The brakes should make noise to let you know when they are worn, but having a mechanic you trust check them out never hurts.