I think it is strain dependent. I have a Nirvana PPP and Female Seeds Pure AK under mine right now and the PPP shows some bleaching any closer than 10", but the Pure AK is fine at 5" (can't raise the light anymore, the AK stretched like a mofo in flower).
It will not go away, so its a chance you take. I would start at at larger distance and progressively move closer, not vice versa. So as polyarcturus states, 8"-18". Start with 18", if you see stretch, move it closer.
I personally would try 12" or so, unless it is over seedlings. 18" at least then, if not 24".
alright. well i have a small tent and put my biggest plant into a 5 gallon pot without thinking and now she is some what close to the light. cant move the light up much higher. i guess ill just have to tie her down alot so she doesnt bleach.