I would assume it was pretty recent..or OP would have smelled it before pouring it into the bowl. I'm thinking OP poured themselves a nice bowl of cereal and then had to answer the door or leave the room for a short period of time. The cat presumably either was jealous that OP had such variety in food or was kindly leaving a reminder that the litter box was full. I'm more curious as to how the cat could use a bowl of cereal as a litter box if there is milk in it.
OP specifies bowl and may have gone to get the milk from the fridge after pouring the cereal. My son uses a huge bowl for cereal that kittens and small cats could use as a litter box.