16-17 is fine but 24 is my preference for veg as more light the more quickly they can grow. Switching to flower can be done at any time it all depends on the size of your grow room and what your style of growing is. You can start them off on 12/12 from day 1 or wait until they are a year old(can you guys imagine how massive a one year old veg plant would be grow under a few 1000 HID lights using DWC......sorry stoner moment) Anyways hope this helps and good luck