You'll use the least amount of energy by turning your air conditioner off completely while you're away and turning it back on when you get home.

Simple physics (specifically, Newton’s Law of Cooling) explains why: Your home's rate of heat absorption is related to the difference between the temperature outside and the temperature inside. The greater the temperature differential, the higher the rate of heat transfer.

If your air conditioner is turned off during the day, your home will warm up as it absorbs heat from the outside. However, as this happens, the rate of heat transfer will decrease because there will be a smaller and smaller difference between the temperature outside and the temperature inside. At some point, your house will reach a temperature at which it can't absorb more heat. (Note that if your house is well insulated, this temperature is still usually much cooler than the outside temperature.) When you return home and turn the air conditioner on, your house will take some time to cool, but the total amount of time the AC is on will be less than if you'd left it chugging throughout the day.

If you leave your air conditioner on to keep the house cool all day, the outside-to-inside temperature differential will never decline and you'll begin a day-long cyclical process in which your house absorbs heat and your air conditioner removes it. In fact, if you were to total the amount of heat that your air conditioner removed throughout the day, it would exceed the amount of heat that your house would absorb if you left your air conditioner off. And while setting your thermostat to a higher temperature will reduce the energy expended to cool the house, you'll still save the most energy (and money) by turning the system off completely.

If you're understandably reluctant to leave your air conditioner off during the day because you prefer not to broil while your AC plays catch-up, consider using a programmable thermostat. Set your air conditioner to start up at a certain time so that your home reaches a comfortable temperature before you return.