Hey Katana, unfortunately it is impossible to estimate how much they will stretch. Sometimes you can have 2 plants of the same strain and one will stretch a lot more than the other one. To be on the safe side, you want to expect them to double their height during the flowering stretch. And don't be afraid to defoliated some of those fan leaves that are blocking light to your lower buds. That will help get you more quality tops on them. Good luck!