It also depends on, the watts per gal (wpg), type of plants and there light requirerments, the K rating of the bulbs, ferts that you add and how old your bulbs are. If your using a standard Flor. or T5 bulbs than they need to replaced every 6 months even if they are working becuase the the gassies wearout after that time. If your using CF bulb than it is every 12 to 16 months, depending on the manufactor. The standard run time is 10 hours as Kogo stated.
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