depends on how much space you have and the genetics you are growing. it varies from space to space or plant to plant.
Sativas obviously grow taller, so could be put into flower earlier to limit their virtical growth.
Indicas will typically be smaller bushier plants and can veg longer.
Both strains are liable to double or tripple in size from the time you put them on 12/12.
So - when you grow room is almost half full of vegging plants, putting them into flower at that point would fill your room by the 6th week.