It all depends on how big a plant you want to finish with.Remember the plant will still continue to grow for at least 2 weeks after she shows flowers(Hairs).Are you going to try and flower out with cfl's?
i am flowering some plants now with cfls i have a big 125watt and quit a few smaller ones. They are around 5-7 inches or so. I topped them so it slowed there growth a little. Idont think i want them bigger then two feet.