Trigger flowering (by switching the lights to 12on/12off) according to your available vertical space. The plant will at least double in size so if you trigger at 12 inches, expect a 24-28 inch plant when it is done. Obviously, if you only have 24 inches of vertical space you must trigger earlier. See what I'm getting at?

It's totally a function of how big you want your plants to be and how fast they are growing. If someone says 4 weeks and the plant is only 6 inches tall because it is growing slow, then you just grow it longer.