Personally I would never leave coffee open and exposed to air/light/heat, unless it has just come out of the roaster. I would advise you to store your coffee in an airtight container, and feed the grinder just before you are about to prepare a brew.
Coffee does go off, even in whole-bean form. As to how long it would take for fully exposed coffee to go off? I dont know, I have never left coffee beans exposed for very long (except immediately after roasting).