first what u talkin bout 21 days?
if its the sog grow, u veg 4 21 days then flower, tell me what u r talkin bout ?and i might b able 2 help u out.
in generall terms there is no exact time limit, i have 18 plants just started flowrin yesterday an some r way ahead of the rest, some showin signs of sex some r not, so theres no time frame as such, but as i said, tell me what u r talkin bout.
peace