This is another tough question because it would depend on the wattage of your light, whether or not you have a air cooled light, with lens and ventilation. To put it simply the light should be as close as possible without burning the plants. For example, if you have a 1000 watt with a lens and good ventilation usually maintained about 2 feet above the plant canopy. But to make sure your not to close feel the leaves if they are warm then your light is too close.