How long should Brakes Last?

[robhamster]

1999 E320 4Matic. I replaced the front pads at 38K mi and the car now has 49500 mi and the computer just came on indicating pad wear. I just looked at all four wheels and noticed the the rears are not as worn as the fronts, though I have never replaced them. The fronts look as worn as my old pads that I removed but I did not take these pads out to see if the wear sensor was showing, but, my question, is 11,500 miles low for front brake pads. This car sees mainly city driving.

Thanks