I have used OEM brake pads and rotors until the brake pads were perhaps 2mm thick. A this point, it seems like the rotors were short too. I suggest to swipe a finger across the rotor surface. If you feel like it's uneven, then perhaps it's time to replace rotors as well. In my experience, it's been the case that on many cars the OEM rotor will not last through two sets of brake pads anyways.