Zombie Pigman don't become non-aggressive, unless they despawn or die. They are even motivated enough to chase down someone into the portal and into the Overworld. You'll just have to stay out of the Nether until they despawn.

How do you get Pigmen to stop attacking you?

You have to get at least 32 blocks away from any angry pigmen. Since all pigmen withing 32 blocks of the pigman you hit get angry, you need to go 64 blocks away from where the pigman you hit was when you hit it.

How long does it take for Pigman to stop attacking?

Zombie pigmen now use a different AI. An aggravated zombie pigman now become passive after 20 to 39.95 seconds, although they continue to attack until the player gets outside their aggressive distance. Zombie pigmen now have an attack animation.