You're probably much smarter or more dedicated than me if you feel this level can be reached in a much shorter time.
Wow, in that case I don't think I've ever worked with anybody who's ever managed to reach "junior level" in any language. What do you call someone with, for example, just a PhD in the relevant field, 6 or 7 years of professional programming experience and several successfully shipped products under his belt. And what do you call the guy that is obviously his junior?
But you can most certainly learn all the basics of writing good C++ code, and interop with great C++ libraries and get real stuff done, in fairly short time. It took me about a month to go from Objective-C to C++ and start publishing iOS apps written almost entirely in C++. And if you don't need that, you can start writing command-line utilities in a few days at most.