Since bulbs are pricey, I think that most people wait until they are completely shot. There is merit though in replacing it sooner. All light bulbs dim and color shift over time. Some will even say that there is a significantly less lumen output after 500 hours of projector use.
If you are still enjoying the display, I wouldn't change it. If you are remembering how the images used to be brighter, or "pop" more, then you may increase your enjoyment by replacing it sooner (or now).