Around this time of year here in Southern California, the sun is up in the day for about 11-12 hours. Granted, it's not shining at full intensity all 12 hours because of sunrise/sunset and overcast conditions... so, I would venture a guess that you should just leave your lights on no longer than a standard photoperiod (between 8-10 hours). Anything longer would just be for your viewing pleasure. Just keep in mind plants need the dark as much as they need the light.
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--Daniel
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