If growing photoperiod you decide when to flip them to flower (assuming sexual maturity). Autoflowers do their own thing but you count ten days or so after first pistils (pre-flowers) appear. So a 60 day plant will flower for two months with a 10 day transition at the front end. This is entirely independent of veg.
edit: all flowering times are estimates and your mileage may vary
Loke @Myfriendis410 said, but with photos the girl hairs can show sexual maturity but the plant will stay in veg until you flip the lights to 12/12. Then around 10 days or so later the flowers will start to stack up.