Light would come on for pads that are worn so low the sensor gets worn off. Then you replace pads and sensor. As for rotor thickness, I'd let them get right to the minimum if they aren't warped.

I have the cross drilled, slotted ECS rotors. They look great and have a great coating (why I bought them) and the price point is good. But mine developed a nice warp/vibration about 5,000km in. I'm just running them out and will likely source a Zimmerman or original BMW coated rotor for the next set. The coating is important and I've had good luck with Zimmerman on all my cars (street and track).

__________________
Daily Drivers:
- 2008 535i, 320,000km
- 2004 X5 4.4, 01/2004 production, 420,000km
- 1997 328i, 350,000km

Track:
- 1996 328i, track/race car, ~300,000km

Winter:
- 2013 Ski-Doo MXZ X 800 E-TEC, trail can
- 2007 Ski-Doo MXZ Blizzard 800 HO
- 2001 Ski-Doo MXZ 600 w/800 engine, exhaust
- 1978 Ski-Doo Olympique 340 (vintage race sled)
- 1977 Ski-Doo Olympique 340E