Natural flowering won't start for some til the days drop below 14 hours. 13 or even 12 for some strains.
Is this an old mother, and has it been indoors before? I used to take my mothers outside during the summer, and bring them all in soon as any started showing signs of flowering. Becuase the plants are used to the intensity of glowlights, they treat dusk and dawn like darkness!
Plants can be trained to flower in just about any photoperiod. Cannabis is just desperate to get people stoned and keep spreading, and if you don't allow it to flower naturally, it will start getting more inventive, hence lowryders and hermaphrodites.