From experience - over a period of around 8 weeks (+/- 1 week) - it gets rather...tricky, to keep going with it after around 2 weeks, but after 3 your body adjusts, and you lose the urges.
In a weird way, it's like detoxing - it gets really bad, but keep going and it gets easier.
Edit: Though, for the benefit of people I have met, and likely will never meet, I shall try this.