@Weedolae , you are getting some great info here, but what you really need is some basic weed growing education.
A weed plant has specific stages in it's growth.
1. Seed It's just a seed, its not a plant, yet.
2. Germinated seed, a seed that has opened and is ready to plant. 2-4 days
3. Seedling is the plant from the time it comes up out of the soil to the point it has it first set of 'real' leaves opened. About a week.
4. Vegetative state where most of the growth happens. 2-6 months Light =18/6
5. Flowering state is when the plant is done growing out and starts to produce buds. 2-3 months. Light = 12/12
Each stage has requirements for food, light, water, etc....
If you change the light, you are forcing the plant to change to a different stage, either 4 or 5. You can never go earlier than 4.