Since they are already very efficient, the cost effectiveness of turning CFLs off to conserve energy is a bit more complicated. A general rule-of-thumb is this:

• If you will be out of a room for 15 minutes or less, leave it on.
• If you will be out of a room for more than 15 minutes, turn it off.

The operating life of CFLs is more affected by the number of times they are switched on and off. You can generally extend the life of a CFL bulb more by switching it on and off less frequently than if you simply use it less.

Source—http://energy.gov/energysaver/articles/when-turn-your-lights

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