M.S.
Those are all good suggestions, but honestly, if it's a serious allergy you will know almost immediately. My son has a dairy allergy, and within minutes of eating anything that has any kind of dairy in it, he gets red splotches around his mouth and begins breaking out in hives, and will soon thereafter throw up. Reactions that might take longer to see would be things like exzema (sp?) or breathing problems. But I would find a list of allergic reactions so that you know WHAT to watch for, rather than concentrating so much on WHEN to look for it. So that no matter how long it takes you know what the reaction is. I would say though, try to just add one food at a time while you're introducing new foods. But then, you can add new foods to ones that you know are ok. For instance if you know that applesauce or sweet potatoes are ok, then you can add berries to the apples and if there's a reaction you know it's the berries...see what I mean?