We use sous vide and other circulation techniques in my restaurant's kitchen all the time and here's my conclusion for you:
Previous notes about the melting temperature of duck fat are accurate. However, your cooking method should depend on the result you're after. Thus, I've provided two methods below.
For a MR breast:
The first step is to circulate your duck breasts at 131 degrees until the internal temperature reaches 131 degrees. This generally takes about 90 minutes. Once cooked, we properly cool and store our breasts (below 40 degrees) until we are ready to serve them. When it is about 30 minutes before you're ready to eat, pull the breast from the refrigerator and let them come to room temperature (or about 60 degrees). At about 15 minutes before you're ready to eat, sear in a saute pan over medium-low heat until that middle layer of fat renders out and the skin is nice and crisp. This should take about 10 minutes. Turn the breast once, long enough for the flesh side to sear slightly and for the middle of the breast to come back to about 122 degrees (this will be warm in the mouth but will not cook your beautifully circulated meat any further). Remove from the pan and rest five minutes before slicing or serving.
For a more traditional confit:
Circulate broken down duck (I like to six-cut mine, two breasts, boned out, two whole legs and two whole wings, bone in) at 150 until the internal temperature in the thickest part of the thigh reaches 150. This should be enough time to render most of the fat, but not all of it. I find that this should take approximately two hours if you start from room temperature, three from cold. At this point, fully cooked, you can cool and store your duck in bag in fat for six months or more. If you're ready to serve, in a medium-hot fry pan sear your pieces on the skin side only until crisp. This will be considerably faster than the previous method and you should be careful and pay close attention to your pan. The skin is easy to burn once it's been fully rendered, much like a piece of chicken.
Good luck & enjoy!