That seems a little fast for a tank. What temperature are you keeping it? That's going to be the most important metric. I keep my oil heat at 55 and use the pellet stove to keep that bill down. The pellet stove runs about 150$ a month and keeps the oil bill usage at like 50-100 gallons a month in the winter. I usually do 750 gallons a year ish. Attic insulation would be the first thing I check if you're losing heat.