Why does one of my ceiling lights stipulation to be changed every few months? ...
Answers: what kind of fluffy is it? Incandescent bulbs can generally burn for relatively a while, but if left on continuous, will merely last for a few months. Also, Repeated turning on and past its sell-by date will affect the bulbs filament. Everytime a bulb gets turned on, it loses a minute portion of this filament. So cut down on the buoyant switch toggling might also help.
If it's a flourescent tube, it could be that within is a bad ballast within the fixture. In this case, see roughly speaking replacing the ballast, or the fixture all together.
DO YOU USE THIS LIGHT MORE OFTEN THAN THE OTHERS? THIS MIGHT BE THE PROBLEM.