Rear Break Pads & Rotors - Mercedes E320 2005

My Mercedes dealership said my rear break pads were currently at 4 millimeters and they recommend replacing when the get to 3 millimeters. If I drive 5K miles / year mostly city miles (not highway). At what stage will break pads last until it will "not" pass inspection? Also, when you purchase a brand new car how many millimeters is the starting point? For example I purchased my car in 2005, I have 73K miles on it...12 years later I'm at 4 millimeters...what was the beginning millimeters? This helps me understand if I have 6 months left or 2 years left or 6 years left.