Take it from someone doing 25 done 12/12 from seed. Here's the FACTS:
If done 12/12 from seed your plant will go through 5-7 days of veggitative growth. After it accumulates enough sexual(flowering) hormones, it will start to flower. At around 16-21 days, they will show you their balls or not. Now, I did bagseed with 50 originals so. I had some be sexed a lot earlier at only 5 inches. While others took over a month to sex and where almost 2 feet at sexing. So picking a good strain for this would seem critical if you are hoping for a good yeild.
They will not only be 8 inches at harvest. That ludachris! It's a weed, and they grow like one! I have litterally tied my girls in a knot before, and they still grow. I had a bud, looked just like a pretzel years back.
I've also done as much research on the subject of 12/12 from seed as can be done. Not to many people doing it out there. Because it is a little of a waste. But from the few I did find they reported around an oz or so from each plant done 12/12 from seed. So, I am aiming for that. With the right strain this is absolutely possible.
It's a little to early for me to tell you a good guess at yeild. But my smallest girl is only 10 inches. I'm expecting about 20grams from her dry. She smells so dank too.
My tallest is at 29 inches. But still early in flower. I stagered the harvest a bit.
No here's my advise: Don't do it. You'll be disappointed. This was an experiment for me to try a lot of different growing techniques. 12/12 from seed was my attempt to rush the expirement. I wasn't expecting shit so I am pleasantly surprised. But if your hoping for weight, this is not the way. Unless you had A LOT of them. I would seriously advise around 2-3 weeks of veg from seed if you wanna be happy with your hard work. That will still accomplish short plants.
If vertical feet is your concern, consider a SCROG. If it's a fast harvest you want. Then look into cloning, and a perpetual harvest may be for you. But don't let me discourage you. Nothing is better than personal expierence. It will still take around 3 months though so you might as well do some vegging. If you have your mind set on it. Look for a good strain suited for the growth your looking for.
Here's some pics from my thread. Also, some other pics from another thread. This guy does the whole cycle in 24 oz bottles. Yeilds a little under an oz dry with 12/12 from seed. So don't believe the hype. You can smoke a girl grown 12/12 from seed just fine. Just not as long as one that was vegged. I haven't heard anyone comment on comparable potency though. Follow my thread and I will, post such info.
Hope I helped ya. Let me know if I can help more.
-The Dude
Here's the pics. My oldest girls are 6 weeks in flower. The rest are following suit. The last two pics are off the other thread I spoke of: