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I would say 8 hours is more than enough. Turn them on when you are home so you can look at your tank. After all that is why you bought it and do all this work.
I turn on the room lights for a few minutes in the morning before turing on the tank light. Then I shut it off a couple of hours before I go to bed so in my case, the tank light is on about 12 hours a day...give or take.