Before you switch to 12/12 there are a couple of things to consider.
Pot size if your growing in soil, make sure your plants are in the pot they are going to be in for flowering; otherwise transplant it at least a week if not two weeks before you flip the lights.
Space your growing in. Your plants are going to double if not triple in size when you flip them over to flowering. So make sure you have enough room for them.
Lighting limitations. CFL lights have to be with in a couple inches to be effective. The larger your plants are the more lights you will need around your plant to get a good yield.
If you have the place, the lighting and a big enough pot that your plants aren't going to get root bound then switch em when ever you like. It's harder to grow large plants with CFL, your better off going with more plants that are smaller and LST/topped/FIM.