Whenever you want really. It depends on how big you want the plant to get. Sativas will get approximately 3 times bigger once you flip, and indicas 2 times. Id suggest topping and LSTing for a while so you can get a bunch of nice colas forming, then flip it.
Yeah, it can depend on many factors. Space, time, style of grow. I have had grows that the clones go straight to budding, or bushing a plant SCROG style and budding when she fills the box.
I usually switch when the plant shows pistills itself, which is usually a month of veg, which is what i did with my plant thats finishing up now (if you want a day by day photojournal, click my link - current thru today). Once she showed pistils, bam 12/12 and off she went running. now i've got 14 healthy, DENSE tops. Its a small plant from my training but what i did, well.... simply worked. On my next grow I am going 24hr light until i have the plant shaped in a way that it can go straight up the rest of its lifespan. i think that oughta work better. i've been fighting mine with LST throughout flowering and im thinking if i can avoid that it'd be better.
Clones can be flowered right away. Seedlings need at least four to six weeks before introducing a 12-12 photo period. Like mrbear2point8 said, you can begin as soon as you see pistols.
its been done, just remember, a plant can only produce as much as its able to support, so you need a solid stem structure to be able to move nutrients and provide growth to the buds. just like any other plant.
LSTing is "low stress training" i think it stands for. There are many different techniques of LST. Most people will use twisty ties or string and bend the actual stalk sideways as to let more of the main branches grow upwards toward the light and creating more budsites. LST could also be just tying down larger branches to let light into the inner ones. Etc, etc. Colas are just the main thick shoots that carry your largest nugs. You create more colas through LST and topping/fimming. Flip it just means switching light over from 24/0 or 20/4 or whatever you had vegetative to 12/12 flowering.
Your plants look really good.....my fear is lighting, I'll be using about 200watts of cfls to grow 3 or 4 plants..... But I was planning on letting them flower and veg for longer since its not an extreme amount of light..... I want to do 12/12 all the way through but I'm worried that my plants won't grow big enough during veg and I won't get a good enough yield.... I'm lookin for about 2 or 3 os from my grow total.... I could use any advice I get