The coldest I've ever commuted in was -28C / -18F a couple years ago. The trip took over an hour and my fingers and toes were not protected nearly enough. No frostbite, but there was a very painful and prolonged warmup period once I got to my destination. I don't ride much in the winter, but I wanted to try a really cold commute at least once. Since then, the coldest I've ridden in is about -15C / 5F.