the pads should have wear indicators on them. when they are gone, replace, or keep an eye on them often.
My rear pads wore out long before the fronts too. I think that maybe the pedal is a bit high, or something in the seating position, etc. tends to cause one to get on the rear brakes a bit earlier, or harder. I try now to apply the fronts first and then the rear when the fronts are just beginning to grab. We'll see if that makes a difference..