not enough info. there are alot of things that go into determining yield it is very hard to guess. lights, soil, ferts, growing tecniques, they all play into it.
Well, you did yourself a disservice by not topping the plant when it was younger. Now you have a really tall plant that needs a powerful HID light or a bunch of LED lights. With the HID light, 600W or 1000W, you will be wasting most of the potential for the light. With LED's you need a light on top and at least one or two mid section for the lower buds. Unless you are a big time grower, with tons of plants and insane lighting; always top your plants. Much better to have an even canopy, so more light hits at an even level, thus more light is absorbed by the plant. Cannabis grows like a tree, optimally you want a bush. Unless you have a massive grow operation where you can't top plants as a time constraint, topping basically is mandatory. Same rule for Sativa and Indica or mixed plants. Even if you are growing outdoors, why would you want a really tall plant, rather then a wide plant? If it does grow outside, it will eventually become a tree regardless. Why not have a larger tree? You loose nothing from topping the plant, it recovers in 3-4 days and has typically 2 or even 3, 4 off shoots growing in it's place. It's insane that every tech doesn't have this as mandatory rule.
Most answers to this question which is posted like every other day consist of explanations on why there are so many factors for yield. However, with a plant that is 2 feet tall and 16 inches wide BEFORE it goes to flower, you should get a very decent yield. If the plant is this tall and wide a few weeks into flower then I would say less than an ounce. If the plant is this tall and wide and ready to flower then you should get at least 2 ounces possibly more. If you have not topped the plant or lst'd then your not going to get much. A plant should have a minimum of 4 tops which result from a two toppings or LST. You can also get 4 tops by supercropping and bending the plant sideways so that the side branches become tops. Supercropping increases nutrient take up by causing a knucke in the stalk which accelerates nutrient flow after the plant has healed. In the past I have topped for 4 tops now I make sure a plant has 8 tops before flower. This usually doubles the yield from a single top which has a lot of vegetation and a fat main cola to many medium sized tops. When the plant starts to stretch to a height where you don't want further growth in flower then prune the plant to let light to the lower branches this shocks the plant and stops it's vertical growth and by the time it recovers it will spend it's time blooming and not stretching. This gives you a method to control height as well as increase yield, but be careful some plants especially sativas can shock for over week from the pruning. This is why I think the 3rd most important quality of a plant is it's ability to to recover from topping and pruning quickly while density and potency of the buds are 1 and 2. Some plants do not respond well to pruning. If this is the case you should consider lst if the plant is worth developing (mom with clones).