When to change Brakes?

I have a 2004 Ford Focus with approx. 50,000kms. After my last oil change at the dealership I was told I needed new front brake pads & rotors. I noticed no squeeling, pulling to the side or shudders while braking, but they told me I had poor wear on them with the front brake pads having 6mm remaining. What does this mean? How many milimeters can be left before the pads need to be changed? I did get them done somewhere else with a certified mechanic and he told me they did not need to be done yet, they still had lots of pad, were not rusting or worn. But I am curious about the 6mm thing.