Depends on your grow conditions and kannabis plant strain.
A example would be White Widow in excellent hydro grow conditions (HID light, temp, nutrients, humidity and more) should only be grown 2 to 3 weeks then switched to 12 hour cycle. Most kannabis plants will grow 1/3 to 2/3 taller in flower cycle so keep that in mind. Some growers have the resources to wait until a kannabis plant matures before starting the flower cycle.

Note: Mature kannabis plant has alternating leaf nodes. Leaf nodes are where the leaf attaches to the branch. Leaf nodes directly across from each other is a inmature kannabis plant and it can still be put in to flower cycle.

Good Growing!