It's your body's histamines going crazy. They've been ramped up from the poison ivy and it's going to take a while for them to settle down. I don't have a specific number of days or weeks, but I am 4 weeks out post-poison ivy exposure and my original and subsequent rashes are healed but every slight tickle, mosquito bite or dry skin is a MAJOR itch. I can recommend Zyrtec for during the day (10mg I think) and 25-50mg of Benadryl at night. The Zyrtec doesn't make me sleepy at all. You may want to first try that on a day you don't have anything to do to make sure it doesn't knock you out for the day.
