if its a long flowering pure sativa would probably start in a couple of weeks there is no preset time to switch, obviously if you start them flowering before they are mature you will add to the flowering time but you will also be able to keep the plant from getting as big you can even start them from seed on 12/12 if you wanted to, if its a indica/sativa cross with alot shorter flowering period and you just want to keep it under 4 ft would probably veg for atleast 4-6 weeks.