Cannabis will generally double its height after the lights are changed over to 12-12 (12 hours light, 12 hours darkness) which signals the beginning of the flowering stage. Some cannabis strains, especially sativa-dominant strains, can triple in height or more during the flowering stage. Some indica cannabis strains may not get much taller at all during the flowering stage, and may instead just get bushier.
In my experience with several different hybrid strains, a good rule of thumb is to generally expect that cannabis plants will double their height after entering flowering. If the plant is growing long and lanky, with thin leaves like a sativa, expect the plant to grow more. If the plant grows low and bushy for the entire vegetative stage, chances are it will continue to grow in the same pattern during flowering and may not quite double its height. If a marijuana plant has grown stretchy and tall its whole left, expect it to stretch a lot after it starts flowering!
During the phase of life known as the vegetative stage (the first stage of life for marijuana), a cannabis plant grows about how you’d expect… like a weed! In the vegetative stage a cannabis plant only grows new stems and leaves, and can grow several inches a day with the added ability to recover from just about anything!
Be especially aware of leaf symptoms, for example: discolored/yellow leaves, or if your plant starts rapidly losing leaves. It’s completely normal to lose a few leaves at this stage, especially leaves that aren’t getting light (which often look like they may have a nutrient deficiency and then fall off, but it’s just your plant cannibalizing the leaf since it isn’t getting any more light). That being said, overall your entire plant should still be lush and green in week 3-4 while your budlets are forming.
Week 1-3: Transition to Flowering