I only use FOOP as a quick injection of nutes if there's a problem with my plant ... I don't think it's intended to be used as a stand-alone feeding system but I could be wrong. In any event, you'd want to start feeding at about week 2 or 3 and, if it's an autoflower, start with only 1/4-1/2 strength to make sure it's tolerated. And if you use this, make sure you spray just before lights out so you won't risk buring the leaves.
Good luck!