How Long Should I Be Working on a Prototype?

Most of my prototypes go nowhere. I have tried to do survival-esque prototypes that have gone in different directions and ended up not really being like what they were gonna be. Sometimes you know right away that a game is working and clicking within the first seven-day prototype which by the way was very rough. It's kind of like an isometric puzzle game so you explore these isometric spaces but there's a lot of ambiguity about where things are. Nayasquest has been on the shelf for years until he found it came back to it three years later and made himself finish it.