Let's see... I believe I used it three weeks before harvest. I added it for 3-4 days, but did not flush until 6-7. If I could do it over...and I'm going too. =) I would use it 2 weeks before harvest, and just let it flush out as part pf the normal pre-harvest flush. You should also make sure that your plant is doing good by then or it'll probably kill it... The buds for sure got super dense inside. This gave them a more solid core early. When the bud would have been getting more solid, it already was, so it just started getting big fast. At the end of my grow I was spraying a superthrive mix and it was lovin' it. I think this was because of how stressed the plant was from the gravity feedings. The B1 in the superthrive helps stressed plants grow better. I dunno if that helps but it's what I got. I recommend the gravity if you are a plant tweaker and are always thinking of ways to make the buds bigger. (that's me)