18/6 is the favourite for the veg period on them, but they will go into flower simply when they reach the right age to flower, they do not need to be forced into flowering. It is possible to 12/12 them from seed but from what i have read it hurts the yield slightly.
I have three Diesel Ryder growing at the moment and 31 days in they are all flowering nicely, but are very short, the smallest being just 10" with the two larger ones at 15".
Hope this helps.