How long does it take a rash due to a penicillin allergy to clear?

< 7 - 10 days: Antibiotics like penicillin can cause different types of rashes that can vary in how long they persist. The classical true penicillin allergic rash is hives and once the penicillin is stopped usually resolves in less than 7-10 days. Occasionally this can trigger more prolonged hives. Other rashes from penicillin not due to true allergy would include small red bumps that usually lasts < 1-2 wks.

3 to 10 days: Sometimes longer, if the reaction is severe, or you kept taking the penicillin for several days after the rash first appeared.