|You probably won't like the real answer to your question because I think that the answer is, "It depends". My experience with the allergic reaction to Urishiol is that there are several factors at play: 1) How "sensitive" you are to the allergen. 2) How fast your immune system can get on the attack. 3) How much exposure you had (i.e. lying in a bed of ivy vs. just briefly touching it), 4) How quickly you realized you had been exposed and, then, how quickly you responded to the exposure. and 5)What body parts got exposed.
That being said, I find that I first start seeing the rash in the part of the body that got the worst exposure, and it usually starts about 3 to 4 hours after the exposure. Then, and this is the part that gets annoying, I get to watch it "spread" for 3, 4, even 5 days before I can conclude that what I now have as a result of my exposure is visible. So, it will be on my legs (especially in the back-of-the-knee area), then on my forearms and face, then my hands, then my fingers and feet, etc.
Hope this helps...