As Image Reaper said, 18-30" is good. Ideally, you want your light as close to the plants as possible without causing (heat) problems. The close proximity allows more lumens from the light to reach your plants.

If you are using a cool-tube or other cooled growing light, you can place the light closer to your plants. My friend keeps his 600w HPS anywhere between 8-20" from the canopy because it runs in an air-cooled hood which keeps the temperature down where it won't harm the plants.