"The stomach holds a little under 2 quarts (1.9 liters) of semidigested food. That food remains in the stomach for three to five hours. The stomach slowly releases food to the rest of the digestive tract. Fifteen hours or more after the first bite started down the alimentary canal (digestive tract, which begins at the mouth), the final residue of the food is passed along to the rectum and is excreted through the anus as feces."
Source: McAleer, Neil. The Body Almanac, p. 186.
As your cereal was fairly lower in GI than fruit loops for example, it would probably take around 4-5 hours until it is "digested".