depending on how much room you have is when you know when to flower.. their are clear indications that your plant will give you if you plan on growing it to its full potential.. but when you want a smaller plant or a smaller grow you have to compromise with what the plant wants and what you want..
It is indeed possible to flower from seedling, with good results, meaning potency not yield quantity.. Though make sure you do not add nutrients until it is grown enough to handle it.. I would say give it about 2 weeks before you add nutes in..
check out my current grow fer some info and pics.. link below.. ---vvvv