I would let it sit for precisely as long as it takes and not a moment longer.
But seriously it all depends. Gravity, temp and yeast all make a difference and it will be done when it's done. If it were I, and fermentation went off well and vigorous and the temp of the fermenter raised up for the first few days and then dropped off to the temp of the room I'm fermenting in, I would wait an additional 3 weeks after that to check gravity and then bottle if it was done.