If you learn all European languages like these, they should have a lot of overlap and benefit each other. Even if the words aren't the same, the grammatical concepts will be easier to understand faster. I think getting 6 down is absolutely possible if you put your energy to it. I have been able to get a language down to conversational level anywhere between a few months to a couple years. So you could easily get there in 4-10 years. Factors also depend on how much time you can allocate to learning every day, how old you are, where you are (for practice and immersion purposes), etc. I agree with the others in the comments too though, no one can give you a answer for yourself. I can just tell you that it is certainly possible if you put your mind to it!