and put the box lid on. Note the time. The milk will need to stay in the box for 14 to 24 hours. During this time the bacterial and yeasts from the starter, will ferment the milk, and produce wonderful things like, probiotics for digestion and vitamins and minerals that help your immune system. Before today’s jar of Kefir is three days old, you can take 6 Tbs. from it and add to a new quart of warm milk. You can do this 7 times, then it is time to use a new starter.