There are a few failure modes for LED flashlights.
Let's start with the LEDs themselves. LEDs have a projected average lifespan from the manufacturer, usually in the 30,000-100,000 hour range. If the light is properly designed and the driver/power supply circuitry doesn't fail it will slowly dim to a percentage of its new brightness by that xx,000 hour mark. Here's a list of those numbers for most cree LEDs
http://www.cree.com/xlamp_app_notes/LM80_results . The projected time to dim to 80% when driven at the 3amp max current is 25,000 hours, if you use the light for 4 hours a day every day on high/turbo then you're looking at around 17 years before the LED dims to 80%. The human eye can have difficulty seeing a change in output smaller than 50% even when comparing two lights side by side. Using it at lower modes most of the time will extend the LEDs life to levels longer than most people will live. If a light overdrives the LED beyond its rated current it may shorten its lifespan to anywhere between a few thousand hours to a few seconds.
Now the 2nd mode of led flashlight failure, the driver circuit. This is the part most likely to fail other than switches on most lights. It's a compact circuitboard filled with parts that basically acts as a power supply and voltage/current regulator for the LED. Soldering quality, defective components, and poor circuit design can all lead to premature failure. Even potting compound to fix the vibration issues could cause problems if its thermal expansion characteristics aren't a good match to the operating temperature of the driver board. A good LED driver should last upwards of 10 years, past 25 years and a bunch of things come into play like environmental aging of the plastics and insulators used in the board components.
Edit: Oops, almost forgot. The real killer for LEDs in an otherwise solid light is HEAT. Especially on the smaller lights, LEDs still put out enough heat that without adequate air movement over the metal body of the light the heat builds up and can damage the LED. Many modern flashlights are being designed to put out the maximum output in the smallest size, which is fine as long as you only run the light in max for a short period at a time. With exceptional heatsinking to the light body the LED will usually be fine, but this is where alot of the cheaper manufacturers cut corners when building lights.(no thermal compound between the LED/body of the light, non-thermally conductive PCBs, etc) Back in the day when Fenix was just starting to get popular they had this issue with their lights on turbo mode, the quality was fine but the lights were just too small to handle the heat buildup when set to turbo and left sitting pointing at what someone was working at for an hour. That learning experience has led to lights having better thermal protection built into the drivers that automatically reduce the output when an overheating condition is detected.
