At least a year? You sound like the kind of person who couldn't diet to lose a few kilos.

My wife needed a few weeks to be able to speak Norwegian on a conversational level, I didn't need much more than that for German (difficult to count exactly due to travelblah). Once you reach conversational level the rest of the way has a different quality — you'll need to learn more but you've conquered the cliff, the rest is a modest incline.