I replaced all 10 in my ATI PM fixture on 2/16/14. I can tell that I have some fading of colors on some SPS's so it's time to change them. So mine have been in 10 months. All ATI lamps.
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They should be changed every 6 months, depending on how frequently they're used.
Otherwise, the par rating drops, as well as the kelvin ( Color temp ).
You'll start to notice abnormal algae growths.
Love my T5's, no matter which lights I go with, LED or MH, T5's are always there to compliment.
Did any one check par on their new bulbs and then checked them again after 12 mo of use to see how much it dropped ?