How long does poison ivy last once it blisters?

Depends on the individual. Conrtary to what a lot of people think, the rash associated with contact with poison ivy/oak is not an allergy per se, but an individuals' skin reacting to the urushiol found in the plant. Those sensitive to it usually see 10-14 days of symptoms. Personally, it affects me anywhere from two to three weeks before finally going away.