On average, water kefir has 3 grams of sugar, equivalent to a green apple. However, the actual amount of sugar is dependent on what you add for flavouring and how long you leave the ferments for.
The standard water kefir recipe is 1/4 cup of sugar per 4 cups water. If you leave your ferment longer, for 48-72 hours, sugar levels in water kefir may be even lower.