It also depends on how powerful your lights are. If you're flowering with CFL's, for example, you wouldn't want a six foot plant. Keep in mind your plants will double or triple in height, after they go into flower. Also - you can start 12/12 from seed - but you cannot force the plant to flower until it's sexually mature. Before that, it will continue to veg, even under flowering conditions. Aside from those few things, it's completely up to you, when you want to flower.