Hi. 2 points
1. The rear pads as new are 2 different thicknesses, one side wears more than the other as there is only one piston pushing. The idea i think is that they both eventually wear out at the same time.
2. When my ABS R1150RT was having its MOT, we put the rear wheel on the brake efficiency roller, and applied just the front brake. There is a fair amount of braking force at the rear when just using the front brake lever, might explain the high wear rate. I get through pads just as quick, and rarely touch the rear brake lever.
But like Timolgra says, for their cheap price, just change them regularly. A quick check for seized piston / sticky caliper pins might help (use red rubber grease for that)