In fact, if you wash with soap and water within 15 minutes after contact, you may not even get a rash. Once you have the rash, it lasts 1-4 weeks. Treatment is directed at the intense itching, and shrinking your rash. Treating the itching is especially important, since scratching can lead to skin infection. The oral forms are probably more effective but can cause drowsiness.
Topical antihistamines are less effective and can cause a rash of their own in some people.
Drying agents like calamine or other lotions are very soothing and speed healing of the rash. They work well, but you have to apply them often. Cools soaks in baking soda, commercial oatmeal or colloidal baths for 15-30 minutes can relieve itching for several hours at a time. They should be used four times a day for best effect. Lotions containing calamine, zinc acetate, and alcohol dry the blistered rash quickly and can speed healing.