So we have a 2003 Audi 3.0. I'm trying to gauge how long brake pads are supposed to last.

30,000 miles - brakes ruined rotors (dealership never told us we needed to in previous under warrantee service)
47,000 miles - brakes changed at dealer again
56,000 miles - need to change now

All the above are for the REAR sets only. The original front pads after 56,000 miles are still at 60%.

Is this normal? The dealership says the fronts last longer because they are thicker than the rear pads.

 

The rear brake pads on audi's usually only last 25k, and if you let the brake pads go metal to metal and superheat the rotors the pads will wear even faster if the rotors were not changed.
Front pads will usually last 50k but should be changed when under 5mm.