There is no hard and fast answer to that question. It all depends on the heatloss of the house and usage. Most fuel is used up getting the house up to temperature from cold, maintaining the set temperature takes less - a bit like a car: acceleration wastes fuel as you have your foot down; once you get up to speed you can ease back on the throttle.
What type of thermostat/programmer do you have? If you are out a lot of the time, an optimizing programmer would be useful as it automatically adjusts the turn on time, according to the temperature, so the house is up to temp when you get home.