You can veg them 24/0 just fine. 18/6 works and so does 16/8. It all comes down to personal preference. Vegging plants don't need darkness. Flowering plants do.
I must disagree. Respectfully of course, but the plants always need darkness. Alot of really important growth is done during the dark phase. Dont believe me? Set up a night vision camera in your room at night. Am i saying it cant be done without darkness? of course not. But i believe that having a dark phase, even for 4hrs will give the plant its most potential for vigorous growth.
Personally, I use 24h light when in veg because it helps me control the temperature of my room. It gets too cool at night and I'd rather not place a heater in the room.
This is an outstanding reason to adjust your lighting. If you need to control your environment, by all means, do what you must to take care of your girls!
Realistically tho, how much of a difference does it all make in the long run? The possibility of a harvest a few days early? why would you spend more on electricity if the benefits are so slight?
On a different note, and to answer the OP's original question, Jorge Cervantes'
Medical Growers Bible states that the hormone levels in a plant started from seed will be adequate for flowering after the
6th week of veg. Any clones taken will keep the same age as the mother it was taken from and can be flowered as soon as it's rooted. Again this isn't the only way it can be done as I've seen many growers go 12/12 from clone, even seed! All up to you.
I like curvy girls so i veg mine 8weeks or more.