Great question for such an important lock-in mechanism. Perhaps a 30-day grace period? No media release so far that I can find references the conditions to keep data beyond the free 5GB Cloud Drive limit.
There is an option to "Download All Files".
Yes, but for how long? What if my credit card expires and I forget to update it? Is there a 30 day grace period for downloading them or are they wiped immediately?
While it doesn't appear to be easily done through the UI, I'm sure contacting Amazon support will get you all of your files back.