They time out after, um, a while. A long while. Not really sure how long - at least ten minutes. And if it DOES time out, just re-equip.
When your Intelligence bonus changes, make sure to disable Canny Defense by wielding something in your off-hand, so that when you enable Canny Defense (by putting your torch/main gauche away), you get the properly-calculated Dodge AC bonus granted by that Intelligence bonus.
I believe the same quirk applies to Amazing Grace.
And remember - Precise Strike and other cool Duelist schwiftiness works because the Duelist has both hands free for jumping, rolling, and tumbling about, tossing the sword from hand to hand. If you find a way to dual-wield and still get your Duelist bonuses, please don't.
While the engine's limitations mean that there may be ways to do that, the abilities were designed to be unusable while wearing armor, or while something is in your offhand. <3
Duelists rule.