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9 months. Afer that the light output falls off real fast and the CRI (color rendition index) is all screwed up and a spectrum change starts in the bulb.
What I find so amazing is that all us salty people know there is a limit to PC bulb life but every time I post the 9 month life limit of PC bulbs on the FW forums I get shot down that I dont know what I am talking about and you can run them till they burn out.
I can tell you there is a noticable different in my PC lights from month 1 to month 9. If I can notice the difference then that means the corals have been noticing the difference for quite a while longer.
Lux meters are cheap and will tell you exactly when to change a bulb. FW doesn't have a lot of photosynthetic corals so they don't have the same need we do.