At minimum you should get a solid measured (not estimated) 2-hours of battery life, but also remember that burn time will always be a compromise between how powerful your light is and the SIZE of the battery pack. Hauling around a 10 lb. battery brick in your water bottle cage is no fun, so unless you really need that 6 hour burn time, maybe you should pass on carrying one!

Also one last tech tidbit; given a similar burn time, Li-Ion (Lithium-ion) packs are always much lighter and smaller than both NiMh (Nickel Metal-Hydride) and NiCad (Nickel-Cadmium). More about battery types >>