I'm not sure this can be answered accurately. What the last occupants used depends on how warm or cold the winter was and how high they kept the temperature. Also, is the hot water supplied off this loop and if so, how much hot water did they use.
Was supplemental heat used? (electric or wood) Many people use small space heaters in some areas such as a bathroom.
BTW, most people do not realize but if you heat your home to 66 degrees it takes almost 25% more oil to heat the same house to 72 degrees during the winter. Those few degrees make a big difference.