How long does duck fat keep in the refrigerator?
If you are talking about duck fat purchased in a store (there might be a use by date on the jar/can) I think at least six months. I have rendered my own duck fat and have kept it at least 2 months with no ill effects. I have seen articles suggesting that as long as there is no mold or rancid smell it is OK to use.
Not sure about in the refrigerator, but I keep a few tubs in the freezer and have used them after over a year without any problems. If I make confit or deep fry with it I will strain it, refrigerate it and try to use it up within a couple of weeks.
This site is amazing. http://www.stilltasty.com/
It has advice regarding duck fat--it says 6 months in the fridge, and 6-9 in the freezer.
It will eventually go rancid, or, especially if you're not careful about using clean spoons, mouldy. If it's neither of those, then have at it!