you can try to flower at 6 inches if you want it will still wait till it is mature enough to flower out which is not how tall the plant is, it is when the plant has a internodel spacing with the exceptions of clones that has already distinguished maturaty.when my veg room is packed i will take a immature plant out to put into my flower room from 24/0 lights to 12/12 to distinguish m/f and sometimes depending on how immature it will take 2 to 3 weeks then if its male its outside for breeding purposes and if female it will be reveged unless i have the room . light stress will be accomplished by interupting there dark hours continuosly for app a week.on/off on/off