I would definately start flowering that baby. The longer you veg for the bigger the plant is going to get. Almost any MJ plant can flower after 4 nodes. Not that that would be a good choice, but you can get an idea of how soon you can start flowering. I like to let them get to be about a foot tall and have a look and feel. Remeber plants grow 2x to 3x the size they are when they start flowering so just kinda feel it out with how much space you have to work with