Those of us who grow ornamental plants year round under fluorescent lights (on for 16 hours a day every day) often change out tubes once a year if we want good flowering on light loving species. Technically, after about 8-10 months (how many hours is that?) the output starts to drop steeply, and then the curve flattens out at a lower output for years before they decide to burn out.
For seed starting one month a year new tubes will last forever. When you are trying to get orchids to flower well you need alot more light than when you are just trying for vegetative growth.
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