It really depends on how ready the plant is to flower man.

If you leave a plant in veg for a long time eventually you will see the sex, no matter how many hours of light you give it. This stage is known as the preflower stage, if a plant has hit this mark it will go into flower almost from day one.

Most people don’t want to or can’t wait till that happens and end up flipping there lights to force the plant t flower before it’s ready and by doing so it can take a few days to a few weeks before you start to see anything really happen.