Chipper,

   From a standpoint of the physics of it, the answer is "a pretty darn long time".  When you remove the ground, the powder isn't going to simply fall off the part.  Will it eventually fall off?  Some of it will but we're talking days and weeks and months, not seconds and minutes and hours.

  From a quality standpoint, the answer is "as short a time as possible".  The coated part is actually retaining a little bit of the charge and airborne contaminants can and will stick to your part.  The longer the part sits around, the more chances for trash or other contamination.  

  Best of luck!

Joey