How long can a 18 month old cat survive outside in this very cold spell we are having?

Someone may have your cat inside their house taking care of it and that is why she can't get to you. I had that happen once. As far as cats surving outside in the cold? Unless your in Alaska they survive. We lived in Idaho with 11 below and the barnyard cats always survived. They will find a way, some warm nook and keep there. Good luck.