I think pretty much all light styles (MH, PC, flourescent) are 6-12 months.
Anenome...we're talking mushrooms, right?
Mushrooms don't need a ton of anything, so you could probably stretch a lightbulb to the limit without too much problem.
As bulbs get old, they can actually exhibit a shift in wavelenght as well as intensity. The shift is toward longer wavelengths (yellow and red) which algea love.
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I will say it depends on the bulbs and ballasts. flourescent on modern electronic ballast, 9 months to 16 months. MH what color temp, 20000k 9 months 10,000 1 year 6500 k 2 years.
