It depends a lot on your body. I ride with just a cheap ($10) pair of windproof/breathable gloves from Kohls down to -20*F and have no real problem. Others seem to need lobster claws at +20*F. At that temp I might still be riding with half finger gloves.

Lobster claws are guaranteed warm. If I wore them at any temp above about 0*F my fingers would turn red from the heat though.