If I understand your question correctly, you're asking when it will start flowering as opposed to how long it will be in the flowering stage before it's ready to harvest?
Cannabis plants (photoperiod ones) start flowering when they receive 12 or more hours of uninterrupted darkness. (If they're autoflowers, they start flowering on their own regardless of light cycles, usually after several weeks of entering vegetative stage.) Once that happens, they'll change from the vegetative stage to the flowering stage. It looks like you're doing an outdoor grow so when your days start being shorter than your nights is when they'll flower. If it's indoor, the grower gets to decide when to start the 12/12 light cycle. Once they start flowering, as others have mentioned, it can take anywhere from 8-12+ weeks before they're ready for harvest.
Have fun!