When it's ready. Really, it depends on plant genetics, grow room size, grow statistics. We can't just put a magic number in front of you.
You can flip it whenever you want. It will grow roughly two thirds the size it is in veg so flower according to your space. You should veg 24/0 unless you absolutely need to shut the lights off. Edit: Maybe I said that wrong. What I mean is the plant can almost triple in size once you start flowering.
yOUR PLANTS WILL never SHOW SIGNS OF FLOWERING untill you flip it over. IT is when you bring 12/12 that it will show signs of flowering...... think of the plants in nature.... they dont start flowering UNTILL their light cycle starts shortening. This is the reaction to knowing winter is coming: so they want to start forming seeds for survival of the species. .......... only males form seeds (cum) ....... if you keep this cum away from your females that will show flowers(buds), they will not be impregnated and not give seeds........................../////////// But to specifically answer you........ switch to 12/12 WHENEVER you want.... if u have small hieghth space switch early....... just expect them to atleast dub in size and some strains could triple. PLAN AHEAD for what your circumstances are.
IMO it will take a month or so of vegetative growth before the plant is mature enough for sex. I typically veg for 30-60 days and then flower. The length of time is pretty much up to you and how big the plant is. Like others have said, the plant will put on a growth spurt (stretch) at the beginning of the flowering cycle so keep that extra growth in mind.
Not true. There are several strains which will show preflowers after reaching sexual maturity. Cinderella 99 will show preflowers around the 5th week of veg if not switched to 12/12. MALES DO NOT FORM SEEDS! Males form sacks of pollen which open and pollinate the females. The females than will begin to form seeds; not the males. You can nduce flowering in any cannabis plant at anytime during it's life cycle; going 12/12 from seed is basically inducing flowering at the seedling stage. Most people prefer to veg for at short time, to sme actually wait it out for signs of sexual maturity; alternating nodes, preflowers, etc... I sqitch my plants indoors once they reach around half of the height of my grow space; if the height is a clear 3 ft, I will go to 12/12 around 16-20 inches.
You guys have more patience than me. This must be the 10th time this week I've seen this exact same question. If people only would put as much time into researching their question as they do creating custom graphic signatures...