Just to add to what Nexus said.
I'd say that a competent developer, who already knows one or more other languages, should be able to become pretty comfortable/competent with modern C++ in a year or two. But I'll never consider anyone a C++ expert who has not used C++ professionally for at least 5 years. The language really is that big and complex. It takes years to master it - and once you do know it well, picking up a library like SFML is going to be pretty simple, but if you don't know C++ well then picking up even a simple library like SFML is going to be a challenge.
So, all I really want to say is; learn C++ well - first - or be prepared to be frustrated often.