How much pad life you have remaining depends on how thick the pads are, their type & your driving style. If you're worried about the extra cost of buying/installing rotors, change the pads & wear sensor now. It's very easy if you have some tools & a way to get the font end of the car off the ground. Plenty of YouTube videos to show you how to do it. Good luck!
2006 330ci ZHP - Silbergrau Metallic / Stoff Laser/Anthrazit / Alu Black Cube trim / Adaptive Xenon headlights / HK hi-fi system / heated front seats