Answer: Some plants will tolerate less light than others, but for seedlings I generally have good results by setting my timer at 16 hours. For houseplants or tropical plants, you can experiment and see how they do with different amounts of time; some plants definitely tolerate lower light levels than others. Also, if there is additional sunlight to supplemement the artificial lighting you may be able to use less. Your plants will quickly tell you ( by stretching or blooming less) if they need more that you are providing.
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