Just got my 02 wrx back from her 48,000km service -- the mech told me the brakes have 40% pad left on them. Question is: at what percentage should they be changed?

thanks in advance.

When they start to squeal. :) It's usually down at about 15-20% when the "squealer" (metal tab on the pads) makes contact with the brake rotors when you step on the brake pedal.

I believe the Suby factory manual says 2-3 mm or about 1/8 inch of lining (NOT including the backing plate). My Forester with 72,000 miles(110,000Km?) has about 25% remaining. I chose to change early as less chance of messing rotors because of heat trasfer.

20% is my spec. Others may be +/- 5%. But none of us uses rulers or any measuring devices to check.

There may be some instances where squealing is heard but isn't caused by the "squealer". Though can be distinguished by individuals who literate on cars for a living, it may be hard for a DIY garage mechanic. To check get the wheels off the ground and locate the back of the metal tab on the pad from behind the rotors and then rotate the tire while depressing the tab. If there is very little play and it starts to squeal upon rotation and pressure, then it's pads. If not, there may be metallic debris or other and simply take them out and scotchbite them (or any other mild abrasive) and clean out other bits. And what I do is then put on some graphite solution over the pads.

-paK +4