What I have read estimated a lot less than that, based on blue prints of the facility and current capacity of hard drives. All of this seems to be pure guesswork, but the biggest number they came up with was in the zettabytes.

http://www.forbes.com/sites/kashmirhill/2013/07/24/blueprints-of-nsa-data-center-in-utah-suggest-its-storage-capacity-is-less-impressive-than-thought/

Regardless, that is a scary amount of snooping being done.