The SCOBY consumes the sugar and produces CO2 (hence the fizziness) and ethanol. The longer you leave the kombucha, the lower the alcohol content will be as the SCOBY consumes the ethanol and expresses amino acids, trace vitamins and minerals.
There is approximately 1-2g of sugar per standard glass of unflavoured kombucha - compared to 24g in orange juice and 13g in natural carrot juice for the same volume. This information is based on one cup of white sugar per gallon (4.5L) (click here to read the web source of this information) - I use fair trade, organic muscovado sugar, between 1 cup and 1 1/2 cups per 3-4L, which is harder for the SCOBY to break down, but is richer in minerals: calcium, magnesium, potassium and iron, and adds a beautiful depth to the flavour and colour. I am also aware that muscovado has a lower Glycaemic Load than white sugar. I always leave my batch for 14 days - you get to know when the flavour is just the right amount of "tart", and you will also get to recognise the smell (a healthy, yeasty smell). The amount of sugar is important as not enough will inhibit the health of the brew, and therefore not produce enough acetic acid - and too much will cause the yeast component of the SCOBY to either flush and take over the bacteria, or fall asleep and do nothing! There is room for you to experiment a little and find out what works best for your climate and ingredient choices.
The pH is important for safety, and of course the flavour and viability of the brew. It should be maintained at between 2.8 and 3.4 (according to Scarlet Aphra in Easy Kombucha Recipes - only $0.96c USD from Amazon Kindle). I just tentatively taste mine. However, I may play with measuring the pH of my brew and checking the sugar and alcohol content just so I can give you some accurate data for the recipe I give you below.
It is yummy after the first ferment, and many people have it this way, but I prefer to do a second ferment - and this improves it even more. My favourite is blueberry and ginger, but you can play with all kinds of preferences. Peach and ginger is another common choice, but you can also use herbs (this will require adding more sugar - see the link to EcoHearth at the bottom of this page for more info on some different brewing adventures) or other fruit. Blueberry and ginger in particular give a really lovely fizz. The second ferment is much shorter - only 1-3 days and is completed in the bottles.