This will depend on the size of your home, or the space you’re trying to cool down. The efficiency part comes from having the temperature at a normal or above-normal state.
An example of ducted AC working efficiently would be a 700 sq foot home with the temperature set at (Programmed) to 75 Fahrenheit or 23.8 Celcius ratio.
There is no need to crank the AC down to 66 when it’s an “Extra hot” day. You must use it wisely. Turn it off when the temperature is manageable, and enjoy a nice breeze.
After all, it is a centralized pressured duct system and should NOT be running twenty-four hours a day and seven days a week anyways.