Typically, brewing time is best at, minimally, 12 hours. At this point, your coffee should have a fairly sweet taste. If you brew longer, such as 24 to 48 hours, the taste will be more potent and bitter. The caffeine content may be higher, though, and the froth will be incredible. As a result, it is usually best to go by your preference here, though most people shoot for at least 16-18 hours for a good middleground.