Can't really say, there are a lot of factors that you need to consider. First what are you growing? Is it a good yielder? How many lumens are they getting. Is it hydro or organic. What nutrient are you using? I had a plant that I flowered at 4 inches, growing in a flood and drain system under 400w hps, when it was all said and done the plant was 8 ft in height and only yielded 1/2 oz of pretty good herb. It was a thai skunk grown indoors. I didn't do any research and just went in blind. Get to know the herb you are growing.