LED lifespans are usually best measured in terms of half life: the time it takes for the LED to fade to half its original brightness.  Half-lives for LEDs are usually quoted in the range of 10,000 to 100,000 hours, but I don't know if these are the real numbers, or just something marketing came up with.  For most purposes, an LED used as an indicator light can be considered as lasting forever.  LEDs used for illumination will need to be replaced eventually.