Powerbulbs.com recommends replacing headlight bulbs on a car in pairs and on an annual basis. The actual length of time needed to replace bulbs varies by the type of bulb and how much the owner uses the headlights.
LED bulbs typically last longer than halogen bulbs, but they are often more expensive. Daytime drivers will normally not need to replace bulbs as often as nighttime drivers, although daytime running lights do affect the bulbs' lifespan. It is recommended to change bulbs before they die completely, as most light bulbs will start to deteriorate and not produce as much light over time.