Hi Marian, hypothetically you could let your brew ferment forever. It would evaporate before it went bad, but it would turn to vinegar long before that. As far as a minimum amount of time, we usually say 5 days. However, it’s drinkable that entire time. Important part here is that the less time it goes, the less strong the liquid you save for the next batch will be — so over time making very short batches will reduce the level of bacteria, yeast, and other good stuff in your brew.