In a single serving of Frosted Flakes, which is 3/4 of a cup, there are 12 grams of sugar. This does not count any natural sugars that may be present in the milk that you put in your cereal, which is usually minimal.
Read the label, if its more then 5g that to many. You can mix frosted flakes with corn flakes to cut down on the sugar. Remember this only applies to 1 cup not a bowl.