you know, the longer you let them veg the better off you'll be. I knew a guy who was an expert grower (RIP - John Jacobs 1954-200

. Which is where I learned what I know as of now. He once told me, that when growing you have to be patient. Thats the key to a successfull harvest.
He's right.
But, to answer your question bro. I would let them veg for about 4-6 weeks, then flip them over to 12/12. You can flip them over to 12/12 whenever you want. Or start with 12/12 straight from seed.
I personally like to keep my grows regulated to a certain size for myself. This way I control my yield (kinda) and I also control how tall she gets. I also use the LST method. That way my babies BUSH out like a fro. Instead of being tall and lanky.