24/0 and 18/6 are the best time scheds for vegging your plants. 16/8 can cause premature flowering and will usually result in a reduced yield, so I don't recommend using 16/8 as a veg time sched.
Once you're plants have a decent number of growth shoots (aside from the top main growth shoot) you are ready to flower. Then it's time for 12/12. When you switch to 12/12 the plant senses this and produces and different type of chemical that switches the plant from veg mode to flowering mode.
Once you have switched to 12/12, there is no going back. However, if you want to re-veg your plant after harvesting her, then that is a completely different story that I won't get into unless you're planning on doing it and need some tips.
Best of luck to ya, any more Q's feel free to ask away.