You can ferment sauerkraut too long, at which point it will become mushy, and the taste will be too acidic, even though the sauerkraut will still be edible. Longer ferments will intensify the flavors and tangy taste of the sauerkraut. The length of your ferment is usually according to your personal taste. The traditional ferment for sauerkraut is about four weeks, but some people prefer a 3-day ferment.
When things rot, the bacteria that cause decay proliferate on the item and begin to break it down. Fermentation is the introduction of good bacteria that inhibit the growth of the bacteria that cause decay. This has the effect of preserving the food item for longer, and even enhancing its nutritional value due to the presence of the good bacteria!