LEDs don't burn out. They get dimmer. After an LED dims to a certain percentage of its original brightness, it is considered dead. The hours they last can vary extremely with various types of LEDs and how they are used.
The types of LEDs used in flashlights and in the lighting industry are very different from the LEDs you see that are used as indicators and such. The ones in lighting drain loads of current (for an LED) and get pretty hot. These ones actually need heatsinks to keep it from burning itself out. A properly designed LED light should easily be able to run continuously for 5-10 years. This rarely happen though since the driver for the LED usually fails before you see any problems with the actual LEDs.
Most small LEDs you see such as those red, blue, and green ones don't need special drivers and don't really generate much heat at all. You could run an LED really hard and end up having it really hot, but that isn't really what they're used for. You rarely ever see these go out.
So in the end, there are only a few reasons an LED would become completely useless and not light up.
1) The driver for the LED dies
2) The LED burned itself out due to bad heat sinking
3) The LED took too much current/voltage and broke
4) A manufacturing defect caused it to die prematurely (I've never seen this happen)
5) Just plain abuse of the LED
A more common defect that was a bit of an issue way back was the fact that LEDs don't always reach the same brightness even if everything is set up the same way. This isn't a big deal anymore.
So here's a common rule to follow for all you people who like to worry out there. Touch the LED. If you don't feel any heat or so little that it might as well be room temperature, then that LED is probably going to outlive any device its put in.
EDIT: I forgot about OLEDs. These ones are organic so they deteriorate a bit faster than normal LEDs. Developers are still working out some kinks for this technology. Don't expect these ones to last forever at its most bright and vibrant state.
Last edited by psycholoner; 10-09-2012 at 01:59 AM.