When you switch to 12/12 the plant will go through a period of very rapid growth called the 'stretch'. In this time the plant may double or triple in size.
If you are growing indoors, you have to start with the max height you can accomodate in your grow area. Find a grow thread for this strain and find out how much it stretches. If it doubles it's size then you veg to half your final height.
You can veg to a larger size if you do some topping and training. Look at threads that talk about FIM, LST, Supercropping & SCROG.
You can pretty much start 12/12 whenever you like. The recieved wisdom is that it's best to wait until the plant shows preflowers and alternating nodes but some growers start 12/12 from seed! There is no fixed time. It all depends on how you run your grow op. From what I've seen 2 weeks to a month is typical. Someone above said a month to 6 weeks. It all works.
Good luck with your grow.