First off. I want to say is never turn your heat completely off when you're away from home thinking you'll save money.
Yes you will save money on the gas bill but turning your furnace off completely will result in ruptured water pipes and water damage to your house costing you more in house repairs and plumbing repairs.
I leave my heat set to no lower than 55 F when I'm gone. Never set it any lower than that. You need to keep your house at least 55 F to prevent frozen pipes.