When you switch depends on:
--How much vertical space you have and what strain you're growing.
A good rule of thumb is that plants will double in height after going to 12-12. Some sativa-dominant plants can TRIPLE in height. So when your plants are a little under half-way as tall as your growing space, you may want to switch the lights. Note that you can reduce overall vertical height by "training" your plants (SCROG, bending, etc).
Some people run 12-12 from seed, to keep the plants as small/compact as possible. This makes sense if you're growing in a small enclosed space.
--When you want to harvest.
Obviously, if you want a harvest at a certain time, you need to start your flowering 6-10 weeks before (again, depending on how long it takes your particular strain to mature).
--How much you want to yield.
The bigger your plants get in veg, the bigger they'll be in flowering, and the higher the ultimate yield (to a point. . .plants will become limited by the amount of light you have and the size of the pot they're in).