That's a hard to say. Usually with an allergy the vomiting is pretty sudden, or in my case it was 2-4 hours later. I've never seen it happen that late, but I suppose anything is possible! You can either bring this up with your doctor, or try her with just straight corn in a week or two, in the morning is the best so if she does get sick it's not at bedtime again. That way if you try the corn straight at least you'd know for sure if it's the corn or something else in the Cheerios.