there is no set time. Anyone who tells you some specific time is someone who can't think past their own needs and experience. With proper light you could veg a plant to 6-8 feet tall over months, or little 20 inch bushes in a few weeks. Of course the strain will play a huge part in it, and oftentimes breeders can give you an idea of how much stretch to expect in flower so you can judge when to stop veg. Mostly people veg to a certain size based on what size they want in flower. Since that depends on your situation, other peoples experiences won't translate to yours
Edit: On the other end it's different though. There is a minimum veg time in the sense that if you put an immature plant into flower it won't flower till it's mature. Maturity is marked by alternating nodes. How long till maturity is dependent on strain and environment,
switching 12/12 before maturity is wasting time since you are decreasing light but not yet inducing a flower.
huh? ru saying that u cant manipulate a plant into flowering by cutting pp to 12/12???