I have looked up the specifications given on ElsaWin for the wear on the front and rear brakes and they are as follows.
Different models have different spec brakes and type no can be found on the options sticker in the service book or in the spare wheel recess. Accordinging to the data given the codes are the first number on the second line of option codes for the rear brakes and the third code on the same line for the front brakes. Both are 3 digit codes beginning with a 1. For example the front brakes on my A3 are code 1ZE.
For the front brakes
1ZF - 280mm discs - original thickness 22mm - disc wear limit 19mm
1ZE - 288mm discs - original thickness 25mm - disc wear limit 21mm
1LJ - 312mm discs - original thickness 25mm - disc wear limit 21mm
1LK - 345mm discs - original thickness 30mm - disc wear limit 27mm
In each case the original Pad thickness, without backplate is 14mm with a pad wear limit, again without backplate of 2mm
For the rear brakes - front-wheel drive
1KQ - 286mm discs - original thickness 12mm - disc wear limit 7mm
1KZ - 286mm discs - original thickness 12mm - disc wear limit 9mm
In both cases the original Pad thickness, without backplate is 12mm with a pad wear limit, again without backplate of 2mm.
For the rear brakes - four-wheel drive
1KF - 260mm discs - original thickness 12mm - disc wear limit 7mm
1KJ - 286mm discs - original thickness 12mm - disc wear limit 9mm
2EA - 310mm discs - original thickness 22mm - disc wear limit - not given
In all cases the original Pad thickness, without backplate is 12mm with a pad wear limit, again without backplate of 2mm.
So given the above figures, the front pads have a wear amount of 12mm and the rears 10mm.
So I assume that if the wear rate is such that it will be down to 2mm before the next time the vehicle is serviced, the dealer will recommend changing the pads. The same applies to the discs. If the owner wishes to check the pads and discs themselves then they may be able to get the existing ones to last longer before they have them changed. But at least the dealer with have fulfilled their obligation to tell you that they will need changing before the next time the car is due in for a service.
On my own A3, which has now done 31,800 miles, the front pads measure 12mm (2mm wear) and the rear pads 8mm (4mm wear), so it would seem that rear pads do wear quicker than the front pads. I know that normal logic would say it should be the other way round. It would nice if anyone can get an explanation from Audi or a dealer. I will talk to the one of the 2 senior technicians when I next go to my dealers and see what they come up with.