you can start whenever you want.. they are many factors to take into account for when to start flowering... usually number one on the list is how much room you have in your grow area.. you have to realize that a mj plant will atleast double or maybe even triple in size from the time you put her into flower... if room is not a concern, you wait for either two things really.. either preflowers, or when the plant stops putting out nodes, branches, evenly.. these two things signal that the plant is reaching sexual maturity and is ready to be flowered...