qflush question

hello i have three closet plant set up with a 600w hps light on them now. they are about two weeks into flower. they are growing in clay rocks inside individual reserevoir. i have been feeding them lots of nutes and the plants are just loven it! THe flowers are forming. BUT... i have never "flushed" my plants with a clearing solution, in this case FloraKleen by Gen. Hydro. I empted the reserevoir of water and cleaned them and but fresh water with 1.5 ml flora kleen per gal in all three reserevoirs . How long do i let them stay in that situation? a few hours like the bottle says, or a few days like the stoner at the hydro shop?
advice?

this is my closet where they lice, this is all three , the pic is puside down, but at this angel, the far top left is feminized sativa mix and match from a dutch seed bank, the one is the middle is from the same seed bank and it is a "house blend" called BDS special, hybird. and to the right is an Master Kush
the second pic is one week into flower (ak49)

is there a need to flush at this point, should i use the flora kleen or just pure water? how long do i let the plants sit in non=nutreated water? 2-3 hours or 2-3 days?