To become a good full stack developer is usually a fairly long process. 3 months is generally the bare minimum, and this time frame is best suited to highly motivated students who are already proficient in front-end or back-end development. The idea is that you can supplement any existing knowledge with some course or experience of any skills that you don't know. For example, if you are a good backend developer, you can take a course which teaches you front-end coding skills. To be proficient in all the stack usually takes some months of experience, we would say 12 months would start to give you a solid understanding.