Time to come out of the box

Ok, I have been growing inside a box for some time now and I am getting tired of it, I feel like a LST god. On my next grow the box is going to the garbage. I plan on keeping my lights. My question is since I have not really grown a plant vertically before, I know that I will run start running out of light as they get taller. Keeping things as equal as I can, where on the plant will it start showing the signs of lack of light first? Will it start stretching from the bottom as I move the light further away from the bottom of the plant? In the past I have LSTed my plants to about 2.5 feet inside the rubbermaid containers. I just need an idea of where to look for problems first once I start growing vertically.