My current grow was 5 weeks from sprout when I went to 12/12 and they were 18-20 inches then. They are now pushing 5 ft. This is in hydro. Anyway, here's the answer to the OP:
1. Know what you are growing, different strains are vastly different in final height.
2. Know your target height due to space, height of lights whatever. Leave some room for error.
3. Assess your horizontal space. Are they far enough apart and do you have room on the sides to accommodate a lot of spreading? If yes then top, maybe a few times. Shoot for a shorter plant with a canopy of main colas.
4. Go to 12/12 when your tallest plant is about 40% of the final height you can handle. Leave the lst until you have some more experience.