It is hard to say...
Where are you located?
What time of year is it going to be? (outside temps etc...)
Do you have other forms of heating that will be left on?
Is any piping in un-heated/poorly insulated areas?
If you are going for a while and not planning on heating, are you going to have the house winterized?
Unless you really need to, I would just turn the temp down to the lowest setting (vacation mode etc) and leave it as is... You should not use too much power doing that....