Replacing incandescent bulbs with ENERGY STAR qualified CFLs right away, rather than waiting until they burn out is beneficial because you can begin to enjoy the energy savings right away. By replacing your home’s five most frequently used light fixtures or the bulbs in them with ENERGY STAR qualified CFLs can save you more than $65 each year. ENERGY STAR qualified CFLs use 75% less energy while providing the same amount of light.
You can save the incandescent bulbs for your fixtures where CFLs aren’t suitable (like in a closet where the light would only be on for a few minutes at a time, or for a dimmable fixture if you don't have a dimmable CFL). CFLs provide the most savings in applications where the light is on for at least two hours a day.