How long should poison ivy last?

Depends: ...on how allergic you are, AND how much of the oily substance (urushiol) gets on your skin. Can be as long as 4-6 weeks. The inflammation can affect skin tone the whole summer. Steroids (oral and/or topical) can definitely help.

Usually a week or so: For most cases, the rash should subside within a couple of weeks provided there is no further contact. Topical steroid hastens the recovery. The most important step would be washing all objects which might have touched the ivy with laundry detergent . Else the ivy resin will keep on causing problems for many months to come.