basically you can do it whenever you want. most people throw it into flowering if they notice space getting tight. however alot of other people change the light schedule anywhere from 2-4 weeks. I think it takes 4 weeks to reach maturity, dont quote me on that though. good luck
depends how big you want your plants to be, if your doing a sog type grow veg it for 1 week...typically flower when the plants around 18"
It all depends on 3 things, lighting, strain and grow space. Sativas you should flower when they are ~3/7th of your grow space because they will strech. Indicas, grow them tall because they wont grow any more. And dont have your plants so tall that the light doesnt reach the bottom.
