A very rough rule of thumb, as much as you can have a rule of thumb, is that you get one night per battery if there's not a lot of other major loads involved. Obviously it depends a great deal on how cold it is (and hence how much the furnace needs to run), how warm you have the thermostat set, what sort of battery you have, what sort of furnace you have, and so on and so forth...but I would expect a pair of batteries to be okay for a single night in most any reasonable case.
There are people here who like to be prepared for most any contingency, and that's not necessarily a bad thing at all. There are also some who seem to like to be prepared for any contingency of any contingency, and for any contingency of any contingency of any contingency, and....
