6 mm sounds right, or 1/4" for those in the US
When the pads get to the same thickness as the backing plate, change 'em out.
You will need to use a G-clamp or similar to squeeze the caliper piston back into the caliper. As the pad wears, the pads get closer to the disk, and the area behind the piston fills with brake fluid, so as you squeeze the piston back in, take care to watch the level in your MC reservoir, it will start to fill up. Also the caliper is self centering, so if there is only one piston, obviously the caliper itself has to move to wear the other pad.
Good time to strip the guide pins, and rub some wet/dry over them.
Dont forget the thin steel shim behing the pads, it stops brake squeal. Its an anti rattle spring, most people thing its the disk squealing, its usually the pads vibrating in the caliper, because of disk "run-out"
To answer your question.....I dont know.