this is all down a number of factors, if the car is an auto it will eat brake disks, if its a people carrier it will also eat disks. as already mentioned if its someone who is heavy handed on brakes and driving constantly round town then they would also wear. my car is now on 90k miles (ive owned since new) and still has the original disks front and back, 2nd clutch due to gearbox failure though. but my motoring is a lot of A roads and not so much round town.
there is a good rule of thumb which ive been using with brakes, 3 sets of pads to 1 set of disks, disks dont last as long now since they removed abstesdos (however you spelk it).
when buying a car i will check the brakes and tires, but i use them as a barganing tool and not a general deal breaker as they are wear and tear items that all mototist have to deal with at one point or another.