when you flip 12-12 the plants will start their stretch phase a few days to a week after, but it all depends how old the plants are,
plants stretch when you flip 12-12 because their is no light, when you have a plant growing in the shade it gets lanky and falls over as its trying to stretch to reach the light, so it also depends on what veg schedule you use, a 24-0 veg will have more stretch when you flip 12-12 then an 18-6 schedule, this is because the plant is not used to a dark period so when you flip 12-12 the plants stretches to find the light again as its not used to having any dark period so it thinks somethings blocking the light and it stretches to find the light,
the reason they stretch is this,
at night plants do other tasks, when the plants in the dark it carries out 3 tasks,
1, it repairs any damage done during the light on hours, so it heals wounds and repairs any damaged stems or branches,
2, it sends energy down to the roots and the roots grow when its dark, this has been proven in tests, plants was grown under 3 light schedules, all where clones and growing in hydro getting the same nutes and temps etc etc, the only difference was the light schedule, 24-0 and 20-4 had a lot less root growth than the 18-6 schedule, so this proved plants grow and develop roots at night, so the more roots the plant has got the more nutrients and water it can take in so the quicker it will grow,
3, during the dark period the plant stretches, again it think its in the shade so it stretches to find the light, when i had a power cut and the light was off for 36 hours, when the lights came back on my plants had grown more in the dark period than they had in the whole week before,
so the night period plays an important roll, so if you veg under 24-0 then the plant will stretch more when you flip 12-12 as its not used to spending time in the dark,
so light schedules can affect stretch,
also age of the plant can affect stretch, a plant stops stretching when its mature and ready to start flowering, all strains will show you their sex under a veg schedule, so if you kept a plant under 18-6 for 6 to 10 weeks then at some point pistols will start growing and it looks like the plant is going into flower, its not its just showing us what sex it is,
plants grown under 24-0 take longer to show sex under a veg schedule,
20-4 also takes a long time but a bit quicker than 24-0,
18-6 shows sex a lot sooner and plants mature quicker due to the night period,
now their is another veg schedule and its called gas lantern routing, its basically a 12-12 schedule but during the 12 hour dark period you give them 1 hour of light in the middle of the dark period and this keeps them in veg,
so the glr schedule is this
12 hours of light
5.5 hours of dark
1 hour of light
5.5 hours of dark, then repeat, the plant will stay in veg under this schedule, i find plants grow much quicker under glr and it also saves on electric costs, plus plants mature quicker and as the plants are under 12-12 the plant is thinking its going into flower but we trick it into staying in veg by the one hour light period, what this then does is lets the plant build up lots and lots of flowering hormones then when you flip to flower it starts flowering sooner and more vigourous.
so its a tough question to answer, it depends on the age of your plant, a plant wont start flowering until its mature and showing sex, so if you see pistils or white hairs at the nodes then the plant is mature and will start flowering not long after, what i find is the stretch last anything from 3 to 5 weeks then the plant will flower, but it all depends on age and what schedule you used for veg,
but if your grow space is not totally dark during the 12-12 flower schedule then the plant wont flower, the plant needs total dark to start flowering, so any light getting into the grow space during the 12 hour light off period then this will stop the plant flowering and will cause it to go hermie