That depends on a lot of different factors: how much time are you willing to devote to it per day? Do you already speak another foreign language? How creative are you in using all available resources to immerse yourself in the language (TV, videos, books, internet, personal interactions, etc.)? Do you have the opportunity to spend some time in a French speaking country?

Basically it boils down to how motivated and how consistent you are. It's much better to spend 20 minutes each day than 2 hours once a week on it.

But you'd be surprised how quickly you'll progress.

-- Chris.