Just like any light bulb, projector bulbs have an expected operating time, called lamp life. This value is expressed in hours and represents the number of hours before the lamp is at half its original brightness.
The expected life of a lamp will vary based on the lamp technology and the projector; however, most projectors offer about 2000 hours. The lamp's success rate is based on a bell curve, so that a majority of (but not all) lamps will meet the lamp life hours specified. Some lamps will fail sooner and this is part of the acceptable operating range of the rating.
For projectors that are used under normal operating conditions (no more than three to five hours per day in a clean, relatively dust-free environment) the lamp will have the greatest likelihood of lasting through its entire rated lamp life