|It's damn tough, and commonly used across many industries where tough, impact resistant clear plastic material is needed. If you search for polycarbonate impact resistance/testing on Youtube, you can find plenty of videos of people beating the crap out of it. Injection molded polycarbonate will have those same properties.
My guess is that most current auto manufacturers are using polycarbonate for OEM plastic headlight lenses, so I'd imagine you can expect these lenses to have similar durability to any new car sitting on a lot today.
Plastic might be more prone to scratches or gouges from sharp objects than our OEM glass lenses, however polycarbonate probably would deal with a blunt-force impact better than glass.