How long you allow your kimchi to ferment depends on what kind of effect you're trying to achieve. Jeong says the best flavor happens at low temperatures temp and after a long period of time (say like two weeks). The longer you allow kimchi to ferment, the more pungent it generally gets. If you ferment it outdoors first, you're fermenting it at a higher temperature so it will ferment faster, but if you ferment it at lower temperatures, you will achieve a better flavor.