The normal time people veg for is bout 4 weeks buts its all up to u bro. u can veg for as long as u like if u want massive plants. the preferd hours 18/6
Well I guess that depends on what you think. In everything I have read and seen it seems that you may get a tiny bit more growth with 24/0 but you see deminishing returns after 18/6. So then the real deciding factor should be is the energy savings beneficial? do you find it convenient that you will not have to cool this area during a lights off period? do you want the convenience/hinderance of having lights on all the time or always observing a dark time?
It comes to preference at some point. weigh your options and savings and see what you decide is right. make sure you are consistant with your lights and do not go from veg to flower to veg again and you will be fine. do at least 18 hours of light for veg and at least 12 hours of dark for flowering (12 hours of darkness stimulates the flowering hormone).