I think 24 hours is normally recommended, but you could probably get away with 18 if you're really pushed.
Is your new freezer frost free? If so, I hope it isn't stored in a very cold place where the temperature drops badly in winter because I've read that frost free freezers were designed to operate in warm environments like kitchens and utility rooms. I posted an earlier thread on this in this section, where one or two people had bought new frost free freezers, stored them in cold places and they'd packed up.