Many of the documents Edward Snowden released are some years old.
Only one year seems strange in light of the new storage facilities being built. The capacity there will almost certainly be orders of magnitude greater than that.
I found William Binney's testimony at this event highly enlightening: http://youtu.be/qBp-1Br_OEs?t=53m24s
In his MIT talk he mentioned that the facility will be able to hold up to 100 years worth of information. I am not sure if it was for Americans or just the sum total of everyone.
He mentions that (conservative) estimate in the talk linked above as well (the question starts at 1:25:16) and apparently that's for the World's communications.