If you are doing an ED or TED, you want to start trialing foods once you reach baseline. This means adding foods back into your diet to see if they are the allergy triggers. When trialing, you should never introduce more than one food at a time. (Trust me... it makes it very confusing when you have a reaction. I had to completely start over on the TED a few times because of this mistake.) Most sources recommend eating at least one full serving of the food for 4 days. Obviously, if you see a reaction on the first day, stop eating the food. If you see no reactions after your 4 day trial, you can assume the food is safe and add it back into your diet.
If you find a 'bad' food - some allergists might want you to trial the food again in a few weeks or a few months. BUT, in my opinion, and in much of the literature that I've read- you really want your body (or your baby's body) to complete heal and let all those antibodies die down. Don't rush to reintroduce foods, no matter how bad you want to eat them again. :) For babies showing a reaction, I recommend waiting at least one year to trial a 'bad' food. The longer you wait, the better chance they have of outgrowing the allergy. (This point is somewhat controversial. There have been research studies done that show that complete avoidance of triggers is the best way to outgrow an allergy, and there have been studies done that show continuous small exposures is the way to go. Either way, I think it's best if the food is initially removed for a period of 6-12 months before trialing or starting small exposures.)
**If you suspect an IgE/anaphylactic allergy to a food, do NOT trial it at home. Trialing of IgE allergens should only be done at a doctor's office!!! I can't stress that enough. Anaphylactic reactions can happen in a matter of minutes- it could be life-threatening before you have time to give Benadryl, an EpiPen, or take your child to the hospital.