You are skilled at fighting with two weapons.
Prerequisites: Dex 17, Two-Weapon Fighting, base attack bonus +6.
Benefit: In addition to the standard single extra attack you get with an off-hand weapon, you get a second attack with it, albeit at a –5 penalty.
Normal: Without this feat, you can only get a single extra attack with an off-hand weapon.
If you use Two-Weapon Fighting on your turn to attack with two weapons, do you also take that penalty on attacks of opportunity made before the start of your next turn? How long do the penalties last?
No, the penalties end as soon as you have completed the full-attack action that allowed you to attack with both weapons. Any attacks of opportunity you make are at your normal attack bonus. Generally speaking, penalties on attacks made during your turn do not carry over to attacks of opportunity unless they specifically state otherwise (such as the penalty from using Power Attack).
Pathfinder RPG Core Rulebook. Copyright 2009, Paizo Publishing, LLC; Author: Jason Bulmahn, based on material by Jonathan Tweet, Monte Cook, and Skip Williams.