Well, really It all depends on how much you like to learn. I suggest learning C++ from websites , books , Videos , Teachers and everywhere internet is full of knowledge.
Also no one can very be best or learn everything in C++, I myself may be a expert in C++ but I can put money right now suggesting there are many if not thousands of better Programmers than me.
Since C++ is very big topic OS to PLC are programmed in C++ and learning all of those will require a lifetime to be a pro at those subjects.
I have been learning C++ for around 2 years and I am able to develop small OS but some people who have 5 years Experience are still not able to program a OS so really it is about how hard you concentrate not how long you learn.
I suggest you first go to google and type "Basic C++ tutorials" just like this: