Okay for both you people asking 'when can I flower my plant' you should go on sexual maturity and age, not height or number of nodes.
Plants reach sexual maturity from seed after about 8 weeks, about the time the nodes start alternating - thats the time to put it into flower.
However, if you have height restrictions in your grow space and area, bearing in mind most Indicas will double in height and most Sativas triple or quadruple in height and hybrids somewhere in between, you need to consider the practical problems of flowering before the 'ideal' 8 weeks of sexual maturity.