you can decide when to flower based on several things -
- when the plant is half the height you want it to finish at
- when you want a harvest within 8 -12 weeks
- when the plant naturally matures based on showing alternating nodes.
most people go by one of the first two methods
as for hermies, you'll see that the same time you see males or females, usually within a couple of weeks of the start of flowering.
if you keep hermies, they will fertilize your females as well as themselves - keep if you want seeds, destroy if you don't