Learning Linux usually seems to be a big deal for most of the users. To be honest, there were a time when it really was. However, the operating system, the user interface and the user community improved a lot since then. developers worked so hard to make the transition from Windows to Linux super smooth by developing window managers that almost look like Windows and fixing the hardware compatibility issues. Nowadays, people started installing Linux on their grandparents’ computers.
Like any other operating system, time to get used to Linux depends on what you are going to do with it and your overall computer literacy.
If you are a regular user
If you are planning to use Linux for regular day to day purposes, you probably won't have any issues other than getting used to the basic look and feel of the system. Only thing you will need to learn is to install the programs to your computer and it is not a big deal thanks to the package managers with graphical user interface. In fact if your Linux distribution of choice has the package, installing the software is easier than Windows or even MacOS. In that case you will be used to Linux in 1 week!
If you are a developer
In this case, the time you will spend on learning Linux is most probably be much less that the time Linux will save for you. Considering you are not interested in some low level coding or understanding the very internals of the OS, you should learn the structure and most of the command line tools in no time. With the help of the online resources and most importantly the command “man”, the required to learn Linux should around 1 week, too.