Well, that's really up to you. When you think they've been vegging long enough and want to put them into the flowering stage, then you switch the light cycle to say 12/12. It's really more dependant upon whether you're satisfied with their size. Keep in mind when you do put them into flowering, they can shoot up to twice their size and even more, so make sure that you can handle it if they get that tall (LST can help with that too as a precaution, easier when they're young, before flowering)

My advice? Poke around the forums and grow faqs and do some reading. Grow logs are good to follow to give you the feel.