you can flower now if you like,i usually start mine flowering after 3 weeks or so to try and keep them kinda short,if you find my grow log you'll see plants that have been vegged for between 3 and 4 weeks before switching to 12/12,they weren't that big when i switched them 16 days ago,but it's amazing how much they grow in flower,3 have about doubled in size over last 16 days and the other is even bigger than that,it's gone from 10 inches to 26 inches in last 16 days,and that height doesn't take into consideration the fact i bent the main stem,i guess it just depends how big you want your plants to be,and remember it's possible for your plants to double or even triple in height during flower