Isn't there another thread about this going on right now?
Graph the outside temp every hour for a day. Then put the inside temp on the same graph. Color in between. That's how much heat you need. If your inside temp dips during "whenever" you save money as it narrows the amount of heat you're adding.
For a setback thermostat to "really work", though, your house needs to coast down to its lower temperature quickly due to bad insulation. Who wants that? Your temperature spread, multiplied by your average "R" value constant, multiplied by cost of fuel, is how much money you'll spend.