It's important to do this right away because the oil of the plant can only be washed off in the first 10 minutes. After that, you can only wash away half of the toxin. After an hour, none can be washed off.
It will take three to four weeks for the rash to go away.
Remedies to reduce the itchiness include a mixture of vinegar and water or calamine lotion. You also can soak the area in cool water with Aveeno Oatmeal Additive.
If it's really severe, your doctor can prescribe steroids in either cream or in pill form. Antihistamines don't help, but Benadryl or a topical hydrocortisone such as Cortaid could provide some relief.