I use a one gallon glass container with 1 cup of sugar for my kombucha. I have tried to tap into it 14 days out and it's just too sweet for me. Mine doesn't taste vinegary at 28 days amazingly enough. The second fermentation is for adding flavorings and to enhance the fizziness of the final product. I like ginger kombucha so I add sliced ginger and let it sit for about 7 days once the fluid has been transferred to a separate container.
Check out the below link. She talks about the kombucha fermenting process including the second fermentation. Essentially, the longer you leave your kombucha, the more sugar it will consume which leads to a lower calorie count. www.thespunkycoconut.com/2012/02/how-to-ma