|Good question Mark,
And unfortunately I don't have a definitive answer but will give you the rough guidelines. The reason that I can't give you a hard number (such as 2 hours) is that the urushiol oil causes a delayed allergic reaction which varies from person to person. On average, if someone has had symptoms before it takes about 12-36 hours. Of course some will be less and some more. For a first time rash breakout, it could be up to 10 days after exposure. I've read a number of different numbers here but I believe the average was about 4-7 days for that first timer.
For me - I know within 8 hours. That darned rash likes to let me know the same day of exposure :-/