When the duck has cooked for two hours, it's ready to take out of the sous vide bath. It's perfectly ok to let it run longer, but you don't want to render all the fat out of the duck--you need some there to get that crisply skin. The duck will be medium rare. The temperature and duration of the bath will kill any bacteria.
Remove the duck from the bags and discard any herbs still in the bag.
Bring a saute pan to medium-high heat.
While the pan is heating, dry the duck with paper towels. Wrap the duck in paper towels to soak up any water that's leaked into the bag during the sous vide bath. If it's not dry, you'll get lots of splatter when you put the duck in the saute pan--one of the things to be aware of when you finish sous vide meats on the stove.
When the pan is hot--probably about five minutes--add 2TBSP of high-smoke oil like canola oil to the pan. ( To test if the pan is hot, add a drop of water in the pan. It will sizzle and disappear if it's hot.) Don't add the oil until the pan is hot. I don't recommend using olive oil, since it has a lower smoke point
Put the legs in the pan, skin-side down, and let them cook. Expect some splatter, so if you have a splatter guard use it. Let them cook for at least a couple of minutes. If you try to turn them too soon, they will stick to the pan, so wait. They'll release when they ready to turn. That's one reason to not use a non-stick pan for this step.
Increase the heat to medium-high and flip the duck. Cook for another minute or until the duck releases from the pan.
Remove the duck from the pan and let rest for five minutes on a cutting board. This will keep oily fats leaking onto the plate and spoiling your presentation.
To plate the duck, swirl some sauce onto the plate, put the duck on top of the swirl, then spoon more sauce over the duck.