Also when driving or braking (could be only for a moment when you brake) if you hear a slight or pronounced squeak you need to take your vehicle to have the brakes inspected. The time for routine brake replacement should be mentioned in your owners manual, but I agree with Mayer it is very highly affected by the way you drive and the environment you drive in. I found that driving in the north I replaced brakes for the same vehicle twice as often as in the south.
Take care having brakes inspected at a dealership. With the downturn in the automobile market some dealers tend to take advantage of those who look and/or act worried. (my mom found this out the hard way when a dealer tried to charge her $450 for a brake job and a replacement disc, thankfully she called me and I told her to call another dealer for their price on having the brakes replaced and a disc, the price was $320) She called the first dealer back and told them she would be in to pick up the car and go elsewhere. So if they give you a pricey estimate, find out exactly what they are doing and shop around between dealers (if that's an option for you in your area)