How long can poison ivy spread?

It does not: Poison ivy does not spread. The rash from poison ivy is caused takes several hours to days to show up. When poison ivy "spreads" it is probably because one is in contact with the oil of the plant, called urushiol, that causes the rash.

Poison ivy: Signs and symptoms of a poison ivy rash include: redness, itching, swelling, blisters, often, the rash looks like a straight line because of the way the plant brushes against the skin. But if you come into contact with a piece of clothing or pet fur that has urushiol on it, the rash may be more spread out.