I can answer that from personal experience: if a poison ivy rash goes untreated, it will go away on its own in about 2 weeks.

If you treat a poison ivy rash, it will also go away in about 2 weeks.

About the only thing you can do is to treat the symptoms and wait patiently. If you can find some jewelweed growing nearby, you can mash it up and apply it to the rash, or else boil it and apply a cloth soaked in the decoction to the rash.

http://www.sciencedirect.com/science/article/pii/S0378874112004382