They show tiny signs at 6 weeks, though you need a microscope lol. But if you want to influence you just create a 11-12 dark period, by covering it with a bucket or black garbage bag, but if you do this you have to continue to do this unless the outdoor dark period is 11-12 hours then it will flower on its own. If you do this your plants might hermie and be garbage if you do not continue to have them on a steady 11-12 dark period till they are done flowering.
PS if its 15/9 outside Light/Night, then cover them for 2 hours at sunset, remove when pitch black 2 hours later. this will create 13 hours of light 11 hours of darkness, they will show buds after 2 weeks usualy.