Do memories depreciate slowly, bit by bit, or do they remain constant for some time and then wiped out completely?  For short term visual memory its the latter:

Which is exactly what happened: Zhang & Luck found that participants were either very precise, or they completely guessed; that is, they either remembered the square’s color with great accuracy, or forgot it completely. It was almost as if their memories behaved like files on a computer: Your Microsoft Word documents don’t lose letters over time, and your digital photos don’t yellow; rather, they continue to exist until you move them into the trash—where they are wiped out all at once.

But for long term memories its the former.  Check out this Scientific American article that surveys some recent research on the shelf life of memories.