[Dryn]

I've used coralife T5 (the cheap ones) for two years and the output is very noticably diminished after only about eight to ten months... I know this because I've got two two-bulb fixtures and I've changed them two at a time since I got them. I'd say they diminish at nearly the same rate as the regular T12 or T8 bulbs, and should be changed every year or so.