Either works, depends on what youre using it for. If only to hold up flowers then week 3-4 in flower; if to spread out growth then earlier. I dont scrog in veg because I have different veg and flower rooms so moving a bush in a 10 gal pot with sceen is not feasible. Also my plants only fill about 1/2-2/3 if the space when put into flower; after they stretch the fill it in completely; I add scrog at week 2 and 4 or sometimes 2, 3, and 4.5 if adding 3rd level