Duck when vacuum sealed will last up to 14 days in the refrigerator but if the duck is not refrigerated then the duck will last around 3 to 4 days in the refrigerator before going bad.
If you want to keep duck longer than 4 days you need to freeze the duck to keep bacteria from growing on it and if it does smell bad and it's only been in the refrigerator then you need to throw it out.
Any type of meat including duck meat if kept longer than 4 days should be frozen instead of refrigerated to keep it longer.