While I do not know what the minimum amount of oxygen needed for good growth is, it is extremely important to ensure plenty of oxygen to the nutrients/rootzone in bubblers. One of the things that seperates this hydroponic method from some others is that the plant roots sit directly in the nutrient solution. Without proper oxygenation of the solution,the roots would quickly drown and the plant die. In addition, the fact that high amounts of oxygen are delivered directly to the roots is what makes this hydroponic method the fastest in growth rate.
Much is known about what the optimum conditions are for growing dope and subsequently, growrooms can be setup to ensure that all the optimum conditions for the plant are met. Once this is achieved,the single limiting factor in plant growth is how much oxygen the root system can be provided. With the DWC method,lots of oxygen is pumped in directly to the roots, allowing the plant to grow at its full potential with an amazing growth rate.
Be sure to supply your nutrient solution with enough oxygen from a proper pump. When purchasing a pump,be sure that it will be able to deliver 500cc's-600cc's/min to your bucket(s) per gallon of nutrient solution. This will ensure that the roots are recieving enough oxygen to be able to grow at optimum rate. One of the best pumps that I have found for a 5 gallon container is the "Elite 802". These pumps have an air/oxygen output of 1500cc/min X 2. For a 5 gallon bucket, this pump would be able to deliver 600cc/min per gallon.