This is a subject with many, many variables. I will need to know what kind of lights, and what wattage. This will determine how close they should be. If you have access to a PAR meter, then I can give you exact numbers to work with as far as how much light they should be receiving. Otherwise, I cannot give you a good answer without knowing some of these things.
Examples: If you have a 4 bulb fluorescent fixture, you will be looking at about 1 or 2 feet from the plant.
If we are talking HID bulbs such as metal halide or high pressure sodium at, let's say, 400 watts then you will want these to be about 3 or 4 feet from the plants.
If I can get this information from you then I will be more that glad to help you out.