97 new corolla brakes (again?)

How long should I expect the brake discs/pads of a 1997 Kudos 1300 to last? I?ve done less than 50K miles in ten years but it looks as though I?m gonna need set number 4! It?s just failed it?s MOT on a 30% brake imbalance and they were only replaced two years ago for a similar reason ( one was alot more worn than the other then). The first set lasted at least five years. Could there be an underlying problem causing such rapid uneven wear? They?re the only parts I?ve had replaced since new and it?s starting to annoy me!