Just a quick update (if anyone is interested).
I've used Defensive Stance a bunch of times now, and here's how it works.
Turn it on during a battle where:
1) You're surrounded by a lot of enemies, or
2) You're fighting a hard to hit enemy.
In these cases, Defensive Stance will last a long time. Defensive Stance will stop when:
1) You move, or
2) You run out of enemies.
My problem was that I used it when I only had to fight an easy enemy, or moved a millimeter to engage another enemy. So it clicked off quickly.
When I used it when there were many enemies, or against an opponent with a high AC (so I missed a lot and combat lasted a long time), it stayed on for as long as the combat took. Which is great considering the boosts it gives one's character.