When your plants go into flowering they will double in size if not tripple. So this means you need to give your plants enough space up top so they wont outgrow there living space. When your plant has filled half your growing area it would be a good idea to switch on the 12/12. IMO when your biggest plant gets to w/e half ur hight is switch it to 12/12. If your topping them though its a different story. P.S. get some more soil in those pots they look low!!