If it has been prepared correctly, that is purified etc in a proper manner, it should last for a very long time...Yes maybe even a year or two.
Because you purchased it, it should have a "use by" or "best by" date printed on the pack.
For food to spoil or go off, it must have all three of - water, heat and air. Remove any one of those and you get into food preservation. In fact, fats and oils, such as your duck fat can be (and is) used to preserve some foods. Coating certain foods in fat will remove the air contact and preserve the food. Some foods are prserved by submerging in fats and oils...A classic example that comes to mind is sun dried tomatoes in olive oil.
So, unless your duck fat has a higher than normal water content, it should keep for a very long time...especially if you keep it in the fridge.
Hope that is of help :)