That's a complicated question dependent on a lot of things. Basically you are asking at what point do you use more fuel- maintaining a constant temperature or lowering the temperature and bringing it back up.
Factors include the R value of your house, the outside temperature, the inside temperature differentials and length of time, the efficiency and recovery rate of the heating system etc, etc. Also, because the outside temperature varies greatly, what might save on one day might waste fuel on another.
A heating engineer could probably give you an better idea than we could.
