How long can it take for a 3 year old to develop an allergy to amoxicillin? She took it for 10 days. She developed a fever on day 6 of the medicine and then started getting hives 2 or 3 days after the 10 days of antibiotic was done? Nothing has changed in her food intake. All I can think of is that she had a virus on top of respiratory stuff. How many days can one allergen exposure take to move through the body? We are on day 4. The other weird thing to me is that they don t show up until mid afternoon. She has taken Benadryl at night last 3 nights but had started Zyrtec prior to this at same time as started antibiotic. Thanks for your input.