You get no warning whatsoever before your batteries die or your bulb burns out. One minute the light works fine, the next it is useless.
I usually replace my batteries whenever I reset my clocks to/from daylight savings time, and replace my bulbs every other time, if it is a light I carry daily and therefore use occasionally.
This is why my most often carried light is my L4. It is about the same diameter as the E2E and only slightly longer, but the LED will probably never burn out. The LED can run on less than full power, and the digital circuitry in the lamp assembly is wired to provide 3 hours of light with an average of 20 lumens after the initial 1 hour at 65 lumens is used up. The slight decrease in the ability to spotlight things at long distance is a worthwhile tradeoff for reducing the likelihood of equipment failure to almost zero.