Im not gonna mention any specific light but I use my led lights ALLOT as a mechanic. And I noticed that at least one with a cree in it keeps getting dim after about 6 months of heavy usage... Is it the heat or something or do all leds just get dimmer over time.
They all speak about 50,000 hour lifetimes but if it dims a good amount in 1000 hours what good is it?
The 50,000 or 100,000 numbers are just single points on a set of curves indicating the 'lumen maintenance' verses current and junction temperature.
There is no single magic number, but rather most manufactures post these curves so you can estimate the drive levels and heatsinking needed to give acceptable life.
The curves are very steep at the extremes, so it is quite possible to damage an LED in only a few hours (or even seconds) at high enough temperatures and current levels.
As far as your experience goes, even a full 8 hours per day for 180 days is less than 2000 hours.
I can't imagine ANY scenario where a mainstream manufacture's light would be so poorly heatsinked that noticeable dimming would occur this quickly.
I can only think of two things that would explain your experience.
1) You are using 3.5 volt lithium batteries to 'supercharge' a light designed for normal 1.5 volt AA or AAA cells.
2) Your problems are actually being caused by oxidation and could possible be improved by a good contact cleaning.
When I see posts where folks talk about using lithium batteries in flashlights designed for simple AA's or AAA's, and they talk about the light running for a short time before the LED 'turns angry blue' (or just dims), I have to cringe because this is exactly the kind of thing which causes permanent damage to an LED emitter.
If you are one of those o' so clever folks [at least THEY think so] that like to use non-approved lithium batteries in your L1D, L0D, etc. and the LED is going to hell after six months, my only comment would be, DON'T COMPLAIN, YOU'R LUCKY THAT IT EVEN WORKED THAT LONG.
On the other hand, if it's a good quality light, using the correct batteries, then you might want to check for oxidation problems, because there is no way it should be dimming after only six months use.