There are three power settings plus "turbo". On the dimmest setting, they are reported to last for about 10 hours. On the brightest non-turbo setting, they seem to last me a few hours (total time, that is. My trips in the dark are usually just across town or to visit friends.) Turbo is recommended for only short periods of time and I suspect that'll burn them out pretty quickly. I think that some folks in the electronics forum have done detailed analyses of such things, so if you need more data, check there.