Typically there is a response within an hour or so. Some symptoms are immediate and manifest upon swallowing the food. Sometimes symptoms only manifest after a certain threshold of consumption is met. Maybe a sprinkling of an offending item wont trigger a noticeable symptom while a bigger amount will. However, symptoms may also not manifest until a day or so later. If there is significant irritation to the digestive system as the food is passed through, there may be symptoms that flare up days after consumption once the body reaches a point where what is going on inside is felt or shown on the outside.