I usually replace mine when I notice the light getting dimmed out, at least with the incandescent ones. I can't remember when the last time was I replaced batteries in any of my LED lights. It seems by the time I need to replace batteries in them, I have either broken the flashlight or otherwise replaced or upgraded it, or else I just don't use it that much. Or...it is one of my wind-up flashlights which just keep going and going and going...