I don´t think it is easy to answer that question because it really depends on your circumstances. When I learned Arabic it was via the immersion method. I lived with a family who spoke no other language so the only way to communicate, even to say something as simple as "pass the salt", it was via immersion. When I was learning Irish the family i stayed with all spoke English as a second language and it took a lot longer. If you are going to learn French or Spanish while still living in the US you really need to have someone you can converse with; if not you end up with that "high school" type of fluency and are more likely to make mistakes/be misunderstood.
A short answer? I think you can be fluent in 3 months (with immersion) or you might take a year (with tapes and a friend) or you might never be fluent (with typical schoolroom type lessons).