ppl usually flower after they reach a certain height......plants can be put into flower anytime but wont sex until sexually mature.... I usually put into flower when plant is around 10-12 inchs.. since they usually double in size in flower.
flower when they get to desired height. take into account that your plant will double or triple in size during the flowering. so if you only have 5ft of space i would flower at 1-1.5 feet.
they will show their sex once you put them onto a 12/12 light cycle. can take anywhere from a few days to a few weeks for them to show their sex.
you count flowering from the time you switched over to 12/12 lighting so if you were 12/12 for 7 weeks then you'd be 7 weeks into flowering. so not when the actual buds start appearing but when you switch the light cycle thats when flower period starts/is counted from. i hope that made sense? lol.