The LED bulb should last virtually forever. From what I have been told a set of batteries lasts a couple of weeks if you use the light for checking IDs, under vehicles, and other quick off/on operation.
And it's built to take whatever you can dish out. All metal parts of the L2 (and all other Pentagon Lights) are CNC machined from aircraft-grade aluminum. This makes for tight tolerances and exact fits for the parts. One nice touch is that the tail cap is pentagon shaped to keep the light from rolling away when you set it on a sloping surface. To help dissipate the heat of the LED light, concentric rings are machined into the lamp bezel.
The L2 also has Pentagon Light's unique tail cap feature. The tail cap offers the user a flash of light with light pressure, or it can be fully compressed for constant light. That means that there's no need to twist and turn the tail cap and only gross motor skills are required to make the light work. The guys I shoot with found this to be a great feature and a big selling point of the L2.
To ensure your light will work in the wettest of conditions, the tail cap and light bezel have double O-ring seals. I tossed my L2 into a bucket of water and let it sit for 30 minutes. When I pulled it out of the water and pushed the tail cap; the L2 worked flawlessly.
If you need different types of lights for a variety of applications, the light bezel of the L2 can be changed with other bezels. For more direct bright light you can swap the head out for the 6-volt Xenon head. If you need to maximize the run time of the L2, say for search and rescue, you can use the multi-LED head, which will give you usable light for up to 48 hours. Other lamp head options include blue light for blood tracking, red and green light to preserve night vision, and a gold lamp to cut through mist and fog. Through this system, a single Pentagon Light can serve a wide range of light needs.