If you wield a second weapon in your off hand, you can get one extra attack per round with that weapon. You suffer a –6 penalty with your regular attack or attacks with your primary hand and a –10 penalty to the attack with your off hand when you fight this way.
Notice that the word "attacks" is plural. You could rule "attacks" to mean only those due to a BAB of 6 or higher, but that is not explicitly stated. To me, "attacks" means all attacks. Since it is also in the same paragraph as the phrase "per round", it also implies that this is a per round set of conditions.
Between that and the similar rule rule (for Power Attack and Combat Expertise, other abilities that decrease the to hit), it is enough to rule that it does extend throughout the entire round.
I also think that because of this, you can get an advantage with two weapon fighting during an AoO that you cannot get with a single weapon: using the appropriate weapon for the job.
If you have, for example, a Cold Iron dagger in your off hand and a Magic rapier in your main hand, you can decide with your AoO whether to use Cold Iron against some opponents or Magic against others.
If you do not allow the penalty throughout the entire round, then you should not allow the character to attack with the off hand weapon on an AoO either.
Shin Okada said:
AFAIK the written rules are unclear about this topic. Some DMs say if you attacked with 2 weapons in your turn, you continue to use weapons in that way until your next turn. Other DMs say you can "switch" your style as a free action at the end of your turn.
If you allow the character to switch the style at the end of the round as a free action, the character should not be able to attack with the off hand between turns.
Personally, I consider allowing this a loophole. If the character is actually attacking with two weapon on round one and round two, there is no "time between turns" from the perspective of the character for which he could switch styles. In my game, if I allowed him to switch styles at the end of round one, he would have to use that style in round two since I would not allow him to switch it back as a free action at the beginning of round two.
This is metagaming and rules abuse. IMO.