At least 2 feet to start...if top leaves on plants act like they are "reaching" toward the light you may be able to move the light a bit closer. If they take a horizontal posture or have that "beat down" look , raise the light a little more. If I'm closer than 2 ft. with my Platinum 600 my stuff goes into slow burn mode. You can do a lot of damage before it becomes obvious, so pay close attention.