Fog lights are high intensity light bulbs, thus their lifespan are not as long as your normal headlamps. Thus it is advisable to use only when visibility is low, such as when it rains, there is fog or no street lamps. You must have used yours a lot as suggested by the 2 previous posters.
Err... do be careful if u turn on yr fog lights in S'pore, hor... i think it's illegal....