How Long Before the 600w HPS Bulbs Lumen Output Decays to Where it Needs Replacing?

I read a guide that said the life of the 600 watt HPS was between 10,000 and 22,500 hours which translates to about 2.5 -5 years @ 12/12.
However, I also read somewhere that the lumen output decays far more quickly than that and that it would be a good idea to replace bulbs after about 1 year because otherwise they are putting out fewer useable lumens and just producing heat and light without 'the juice'.
Does anyone have any idea about this, please?

63 views and no ideas yet?
Thanks to everyone for looking anyway!
I would have checked in the sticky '101 Questions about Lighting' but the 101 questions is spread out over 53+ pages and that makes it a pretty ginormous effort to locate about 80 of the questions!
Maybe, I'll go and ask there too...

hey man, had to reply, as i wondered this same thing a while back. Yes hps bulbs do decay after a fair amount of use, the first 4-6 grows with 1 bulb should be fairly consistent, untill after that , so yea around 1-2yr mark id say is safe, brand new is always gonna be better, look at buying a pack of 12, bulk buy is cheaper. i grew 4 grows with the one bulb, and off 1 plant i got 1.5 pound, after 4.5 weeks grow, and 7 full weeks flower time.

hope that helps man

had to post a quick pick, at about week 6