Rosetta Stone is an enormous waste of money. It's glorified flash cards. Even the theory behind their "learn like a baby" approach, or whatever it's called, is pretty much bullshit in my opinion. If you go through the whole program you'll have a tourist-level understanding of basic Spanish vocabulary, but you won't be able to really be creative with the language and truly speak it to express yourself in anything more difficult than extremely common, everyday situations. Don't get me wrong, for some people all they need is to be able to navigate their way through tourist traps for a couple weeks. But there's no reason to pay $400 for it.
There are numerous, better ways to learn a language. Especially Spanish, considering the immense amount of resources available to English speakers.
*edit* I just noticed:
I won't say that's impossible, but it's an extremely ambitious goal. Do you speak any other foreign languages? Which ones? Have you had any experience learning Spanish before? It also depends on your definition of "fluent". What do you plan on doing with the language? Traveling? Working? Studying abroad? Using it in your current job? All of these require different levels of fluency. If you are really serious about learning Spanish, my advice is to set a more concrete goal that clearly defines why you're learning the language.goal: fluent by September
Also, and I can't stress this enough, start getting exposed to the real language as much as possible. If you have Spanish-speaking friends, start talking to them as soon as possible, even if you can only say a handful of words. Use Skype and start chatting with people, even if it's only via text at first.