I used to take clones from my plants when they were 10 days into flower or less. I did notice effects on the donor plants though, they would slow down and not get that big burst of growing you usually see in the first 2-3 weeks of flower and i didnt take many clones off of the donor plants, 3 at the max per plant. I generally take clones 1 week before I go into 12/12. This works for me but might not work for everyone. Good luck.