Depends on the strain and genetics of the plant and how tall it gets max, you can leave it to veg all you want, it will come to a point where it would stop growing due to 2 conditions, genetics, and the space and size of your pot, you can also leave it to veg as short or long as you want, depends on your grow space.
Veg your plant until it has used up half of the total growing height, then switch to flowering. When you learn how much your plants grow at the stretch, you may be able to veg more than half the height available if the plant has a short stretch period. PW
Yeh all strains stretch different amounts. And half is a little keen as sometimes it can stretch more than half it's size. If your planning on growing a strain more than once first time see how much it stretches when you flip then you can get it perfect the next time on how long to veg for you can keep the stretch minimal if you keep your lights close as you can without burning the slags haha
i veg until pre flowers and then go shortly after that. can be 6-8 weeks but nothing is set in stone.
I have only grown a particular strain once and then try another different strain. But I veg until the plant fills the container area above ground. Then flip.. 3 to 4 weeks avg. And most times they double in size. Just my experience.
If you don't do any training a plant can triple in height when you change to 12/12 . But if you have many branches it can sometimes only grow an extra 50%. There are other factors too, but I would recommend you grow the plant until it has taken up half the available height and then change to 12/12