How do you arrive at the hour ratings for your bulbs?

All electric lights have a published rating for expected life. This rating is in the hundreds of hours for many incandescent lights, and in the thousands of hours for our fluorescents. Fluorescent lights have a life rating based on how many hours they are left on every time they are turned on. This is usually referred to as "burn time", and for fluorescent lights the burn time is three hours.

Every time a fluorescent light is turned on, a tiny amount of the coating on the electrodes is burned off. Eventually, enough coating is burned off, and the lamp fails to start. Longer burns extend lamp life. If you "burn" your fluorescent lamps shorter than 3 hours per start, you use up your potential starts faster and the bulb will last a shorter time period than the published rating. If you "burn" them longer than 3 hours per start, you use up your starts more slowly and your lamp will last longer than the stated hours. However, you are paying energy costs for the operating time of the lamps, and the most efficient lamp is the one that is not on when it is not needed. There is a point where the amount of money you save from turning off the light exceeds the cost of reducing lamp life by more frequent starts. If you pay $0.05 KWh, the time is about 15 to 20 minutes for that point. As energy rates go higher, that time becomes shorter. If you pay less than a nickel per kilowatt hour, your turning-off point would be longer.