I don't really know exactly but I think about 3 months to really get the hang of it. I was familiar with programming when I started so it was easier. Unity has done a good job with the GUI part, this makes it easier to script for the people how don't know programming. I think it becomes easier after you start understanding the basics and how the Unity functions and classes are constructed. Documentation helps a lot. Even now, sometimes I need to check something. I would say Unity is the best beginner friendly yet a powerful engine. There are plenty of documentation and tutorials so it won't be hard to get going.