Its up to you really, but minimum will be about 1month, and its really up to how big you want your plant to get. Remember plants usually double their size in flowering, so estimate based off of your grow room size. Hope that helps
yes what McTwist said, however, depending on the strain you could actually triple in size. My current grow was switched at 13"-15" and are presently 45". Now if you add in the container, another 15" for container and I'm out of room with a total height of 6'8".