10,000 hours of use is generally given as a guideline for how long it takes to be quite good at something.
I'm not great at languages, but I did honours German for junior cert, honours Irish and French for leaving and Spanish in university, so I'm ok at them.
I've lived in France and Germany for a year and a half. In France, I lived in a house with 3 others, none of whom really spoke English, I never had a full conversation in English with them (but plenty in French, naturally). In work, I was hosting technical meetings in French and leading a team of French people, I'd start every conversation in French, sometimes we'd switch, sometimes we wouldn't.
But despite that, I'm not fluent in French. I rarely have much of a problem speaking it, but fluent is definitely going too far.
But to actually be able to speak enough of a language to speak and understand at a basic level? Well, I did a Michel Thomas Italian CD before going there a few years back. 8-9 hour course (but takes 15-20 hrs), I was able to handle myself at a very basic level.
Of course, having done Spanish first helped massively.
It's pretty much all gone now though. Use it or lose it.