How long you ferment is really a question of taste and personal preference. When you are beginning I would encourage you to experiment. A lot of sources I’ve read say you should start tasting after 5-7 days. Taste your Kombucha every day after the first week of fermenting and see what you think. Keep in mind that the longer you ferment the less sugar there will be so if that is a concern for you you should ferment longer. The maximum fermenting time is usually about 4 weeks.
Personally I like the taste after 9-10 days. The amount of time you ferment can also affect secondary fermentation but I will discuss that later in this page.