When shall I switch to flowering?

[Organic Weed]

Hello Folks
I'm in my 27th day of vegetation, 34th from seed.
My plants are already in my final pot (18 litters/5 gals). I am using a Subcool's SUPERSOIL version for my soil, hoping never need to fertilize.
I have heard that the best to switch to 12/12 is when plants have developed pre-flowers. But are these my pre-flowers? How much of the plant shall be covered by pre-flower? How long shall I wait?
My strains: Northern Light + White Widow + Kosher Kush
I did an LST pretty radical (at 90° and keep the plant parallel to the soil since then)

 

[JJ Bones]

That just looks like new growth to me.

Really you can flower when you feel it's appropriate, no perfect time other then what fits your setup.

 

[Organic Weed]

well, they may look small coz the pots are big (5 gals) and the main stem runs parallel to the pot. The stem is only about 3 inches up from the ground and then runs parallel to the soil for another 6 inches. I LSTs and super-cropped them about 2 weeks ago. Since then, they didn't really got much longer and stayed at about 9 inches (main stem) but developed lots of new branches that are growing at about 1/2 in per day. Is that average length/development? Or are they really small plants compared to average? My internodes are 1 inch max. I must have 8 internodes

My set-up is 250HPS agro (no CFL in vege, which my have slowed a bit their growth)

 

[Antics]

This is what a preflower will look like. The taller green tube shapes you see with the pointed ends are called Stipules.