It takes an hour, and it takes a lifetime. For our purposes here at The Forge, we do have material broken into chunks that can be trained to proficiency over a significant amount of intentional practice. Usually it takes a few months of training to become comfortable with the wide range of skills we work on.
What about you? What does “self defense” mean to you? What are your goals? How proficient do you want to be? What kinds of things do you need to defend against? In the end you decide how long it takes.