The quick answer might be something like, a couple of days. Which, may not be too far off. The issue with potato dish preparation, shelf life and how-far-can-I-prepare type of questions lies not so much in the potato chemistry, so much as what limits reside with the added ingredients in a potato preparation. Specifically ingredients such as cheese, butter and sour cream (common in cheesy potato recipes) will limit how many days out a dish can be safely prepared, and assuming all other critical control points are observed (i.e. proper refrigeration). Given these caveats, again leans towards perhaps preparing the dish just a couple of days out, at most.