Hello, It's my first grow and I'm having a one plant of orange Kush, now it's almost 15 inches. I've heard that during flowering time the plant doubles in size, if that's true, then i should switch to 12/12 soon because I don't have much room, but I'm afraid that the plant maybe not mature already to flower, again only 14-15 inches. help!
Its partially true the plant will double the size but its all depending genetics. I have a White widow plant and its about 1 1/5 ft tall and im going to switch it to 12/12 on 420.
If space is an issue/concern, you can always top your plant, wait a couple days and flip to 12/12. A 15" plant (depending on genetics) will be about 36" tall minimum when flowered. If untopped, you can have a "shooter" that gets much taller than the rest. too. If that happens, just pull it back down and tuck it under something else to keep it at the correct height.
You could also try tying your branches down with soft string or paperclips. The side nodes will begin to grow straight up creating more tops. This is a good low stress way of maintaining plant height within a small amount of space. Additionally this could also allow you to even out the height of your canopy. With this your lighting will be more effective and could potentially help your end results. http://forum.grasscity.com/index.php?/topic/1290525-400w-MH/HPS-mixed
Oh I forgot you could also take this opportunity to take a clone for later. Top the plant long enough for a decent cut and tie those branches down. http://forum.grasscity.com/index.php?/topic/1290525-400w-MH/HPS-mixed
I've read and heard to wait for preflowers or alternating nodes. An immature plant will still continue to veg under 12/12 until it reaches maturity.
If the plant(s) haven't shown any stunted growth or issues, it doesn't take more than a couple weeks for it to become "mature". After that, you can flower at any time. I like 5-6 weeks veg in my cabinet. In an open room, I veg 8 weeks+, but that's me.