lol- you can force marijuana to flower at anytime. some people have a 12/12 light cycle from seed. when you do flower, your plant will grow about 1 1/2 -2 times the size it was in vegitation. that being said, if you let it get 12" tall, then flower, she'll be 18-24" approx. at harvest. so it's dependent on what you want and the strain. if you dont know the strain, then the choice of when to flower is up to you. consider factors such as security, lighting, container, etc before making your decision. good luck.....