On front wheel drive cars, approximately how long do the rear brakes last?

I have a 2004 Impala base that has 4 wheel disc brakes. I got the front brakes replaced at 71,400 miles (I got them at 47,800, they were not that bad, but over 50% worn so I just decided to replace them because my mechanic did charge too much, I did a lot of city driving). My REAR brakes were replaced at 47,800 and they have not been replaced since. My car currently has just over 80k on it. I know the rear brakes wear out a lot slower than the front's. How can you tell if you need new brakes (I have alloy wheels, so I can see the brakes)? On front wheel drive vehicles, how long do the rear brakes last or is it based on how you drive? Thanks