The simple answer is that they don't.


It all depends on how many mA the LEDs draw (with some lights now using Luxeon Star super LEDs they can draw quite a bit more than a traditional LED does), how many of them there are, how many batteries your light holds, and how efficient the circuit is inside the unit.

Other factors like a 'low intensity' setting will help stretch life out too. There are many online calculators that can help you do the math on LED circuits. My favorite is here.