How long before a rash breakout after contact with poison ivy?

I don't know the exact scientific time limits but I can tell you from my personal experience. Personally I start to break out between 24-48 hrs and not before 24. Then it seems the rash does not fully mature until 72-96 hours. It all depends how sensitive your skin is and how much of the oil you came in contact with. Some people can break out within a few hours. But I'm pretty sure if you don't break out within 72 hours you won't break out at all. Hope that helps.