Many homeowners wonder whether it saves energy to turn their lights off every time they leave a room. The answer depends on two things: the type of lamp (the technical term for what most of us call light bulbs) in your fixtures, and how long you'll leave it off.
If you are still using old-fashioned incandescent lamps, then you should shut them off whenever you'll be out of the room for at least 5 minutes. Experts from Lawrence Berkeley Laboratory advise that fluorescent lamps are different, whether you are using the long tube-type fluorescents or the compact fluorescent lamps that screw into standard light fixtures. They suggest turning fluorescent lamps off only when you won't need them for 15 to 20 minutes. The recommendations for these two types of lamps are different because the lifespan of incandescent lamps isn't affected by the number of times they are switched on and off, while the lifespan of fluorescent lamps is slightly shortened every time they start up.
These recommendations are based on an average electricity cost of 5 cents per kilowatt-hour (kWh). If you pay considerably more than that, then it may be economical to shut off your incandescent lamps if you'll be gone for 3 or 4 minutes, and to shut off fluorescent lamps if you'll be gone for 10 to 15 minutes.
These recommendations also account for the varying life span of fluorescent lamps, depending upon the number of hours they are used per start. For example, if you use a fluorescent lamp for 3 hours per start, it will last for about 20,000 hours; if you use it for 6 hours per start, you'll get an increased lamp life of about 24,000 hours.
You may have also heard that switching off a fluorescent lamp doesn't save much energy because the savings are erased by a surge in current when it is first switched on. This isn't really true: there is indeed a startup surge, but it lasts only a fraction of a second and the energy consumption during this small time interval is negligible.
For more information on lighting, see Residential Energy: Cost Savings and Comfort for Existing Buildings Chapter 7.