Kaylie answered the bulk of your questions. I'll answer what was missed.
For most people who have been exposed previously to poison ivy (i.e. are allergic), the process by which urushiol oil bonds with the skin can occur in as little as 2 minutes. The average is probably closer to 5 to 30 minutes. Urushiol oil which is found in poison ivy binds below the top layer of skin and creates a protein that is viewed as a foreign invader by the immune system. The reaction is delayed unlike an allergic reaction to say a bee sting.
Washing up 2 or 3 hours later can't hurt as there may be additional urushiol oil which could be washed away. BUT the initial reaction has already occurred.
Your best defense is to be aware of the plants around you.
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