As my livestock population reaches max I can see where CO2 would no longer be necessary, but until then, how much is needed?
The answer to this sort of depends on how much you might require, i.e., what is your ultimate goal in plant growth. Pardon me if what follows is already known to you; but I am one who believes that understanding the processes and how things work is extremely important in order to be able to decide if this or that is or isn't worth it.
Plants require adequate light--intensity and duration--and 17 nutrients in order to photosynthesize which is how they grow. If any of these is lacking, photosynthesis slows and may even cease. Aquatic plants have very little control over photosynthesis; they will photosynthesize to the max, which is up to the point where something is no longer available. Adding CO2 will have very limited improvement in plant growth if the light is not increased to balance and the other nutrients are similarly not increased to balance.
There is a lot more natural CO2 in a fish tank than many realize, and most of it comes from the bacteria, not the fish; just as bacteria are the largest consumers of oxygen, not the fish and plants. If CO2 balances the other 16 nutrients and the light is adequate, the plants will photosynthesize.
I have maintained plants in an aquarium with no fish, and no CO2 diffusion [I also am not one who adds CO2 because I prefer a simpler more natural system], and the plants remained healthy, though they did not increase as rapidly as they do in the fish tanks. The point here is that there is a lot of CO2 in even this sort of tank. Adding it means increasing the light and other nutrients proportionally, which may or may not have a benefit in your case. Which brings us back to the initial pint, what your goal may be.
One comment on CO2 at night--don't. There is absolutely no benefit to adding CO2 during darkness, as it can only be detrimental to fish and plants. It is not going to be "stored" in the tank, it will drive out oxygen (if CO2 comes in, oxygen goes out, and vice versa), and cause problems for the fish, bacteria and eventually plants.