As long as the plant is sexually mature you can. You can tell when the branching of the nodes start to alternate vs branches growing directly across from each other.
I’d add that you can flip to 12/12 anytime you want. There’s some people that grow their plants 12/12 from the very start if they are growing in a tiny space, or don’t have much light. Granted they don’t yield a ton, but you can do it.
A three foot wide plant unless you tie it out will stay close to 3 ft wide, as all growth will be towards top. Some plants will stretch alot but that depends on light intensity and spectrum. You can train and veg a long time and grow a monster. I suggest you try to train outwards and keep as short as possible by tying down. Flip when she is healthy and growing well for best results. Dont flower sick plants.