FIM them now, wait about 2 weeks and flower. You should really wait about 4 weeks after FIMing to flower, but it wont hurt that bad to do it in 2. After 12 inches a plant, grown from seed, is mature enough to flower. They will get about 36 inches tall when done depending on the strain if you flower around 12 inches tall. If you FIM you will more than likely get a larger harvest. Are you using flos? or HIDs? laterz..