The cost effectiveness of when to turn off lights depends on the type of bulb and the cost of electricity. The type of lightbulb you use is important for several reasons. All lightbulbs have a nominal or rated operating life, which is affected by how many times they are turned on and off. The more often they are switched on and off, the lower their operating life.

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