I agree too many variables, but you can get a ball-park number by looking at your battery(s) and taking the total reserve amp hours and dividing by 8 amps. This number assumes the furnace is running all the time (100% duty cycle- worst case scenerio). If you assume it will only run 1/2 the time ( 50% duty cycle), then mutiply the number by 2 etc ..etc.
From practical experience, my batteries (two 6 volt in series) will run the furnace for 3-4 days in below freezing temps ( at night 30 % , Day 15% duty cycle) before needing a charge.