Really the veg cycle is pretty much up to you. I've seen people start to flower really fast. I personally would not bother untill at LEAST six inches, but beyond that, the bigger they get before you flower, the more bud you will be able to harvest. Of course this takes longer. I've seen many setups with two rooms, one grow and one flower, and so each plant got 8 weeks of grow, then moved into the flowering room for 8 weeks for flowering. He got about an ounce off each plant. Many MANY things can change this, but I let 'em grow for as long as I can stand it, factoring in the 8 weeks. My current grow has been going for about a month, and I'm expecting at least one more, since I have the room and want lots of yummy buds