How tall can grow Durban Posion in late flowering?

My past experience with durban poison as far as stretch, is that you can expect at least 12"- 18" stretch in flower stage. More light might not help if you don't have a buffer space between the canopy and the lights. If you have the room then by all means add more light. And as mentioned the buds will really begin to fatten up in the late stage flower.

Durban Poison can grow up to 90” in height, as far as bud fattening that comes with time, normally the plant gets most of its bud weight in the last few weeks, so if the light you've been using is working it should be fine.