It is all based on how tall you want them. A 12 inch plant will be around 20 to 36 inches, or you can top the plant and let all the side buds get bigger to keep height low. My current plant was 11 inches, put it into 12/12 and is now 30 inches, normally will grow in height for about 2 weeks(after pistols show) into 12/12 then with get side growth. I have seen zero height growth since the second week of 12/12 and showing pistols, with this and my last plant.