In reply to Ralphie_coombs:
-10 DegC, in case a snow hole isn't possible and you end up bivying in cold, damp and breezy conditions, and may be out serveral nights in a row with potentially condensation decreasing the bags performance night on night.
Only a rough guide, as everybody sleeps differently, and how warm you will be depend on far more than just the bag and the person. Feel free to decrease if you sleep warm, always carry enough food to eat like a horse, don't intend to be physically worn down, have a thick matt, will change out of damp gear, etc etc.