The point that you switch your marijuana plants light schedule to 12 hours of light and 12 hours of dark to induce flowering is up to you and depends on two things:
- The size of your grow area
- The average height of the strain you are growing
Some people start their plants on 12/12 from seed almost like an auto-flowering strain, while others let their plants grow to be 6 feet tall before they switch to 12/12. Everything that I have read says that you should wait until your plants get to be at least 24 inches tall to start them flowering… and that is a good standard minimum height. Obviously the longer you let the plants stay in the vegetative state the more your plant will yield in the end. If your grow area is small you will have to gauge when to start your plants on 12/12 by the amount of space you have and the type of strain you are growing.
The bottom line is that you have to decide when to start your plants flowering from experience. If it is your first time growing, go with the minimum 24 inches tall and get some experience using your growing area and you will be able to better judge when to start your plants flowering.