In most interviews, William Binney estimates Utah will have one zettabyte of storage. I may come back with a source later. He worked very closely with these systems in their infancy, and estimates they can store 500 years of current data at maximum... but will most likely not use one large volume, and in actuality they can store 100 years of data written numerous times so they can parallel process for cryptocracking.
nl|12 years ago
The NSA's Utah Data Center will be able to handle and process five zettabytes of data, according to William Binney, a former NSA technical director turned whistleblower.
http://www.npr.org/2013/06/10/190160772/amid-data-controvers...
However, from the same article:
Despite its capacity, the Utah center does not satisfy NSA's data demands. Last month, the agency broke ground on its next data farm at its headquarters at Ft. Meade, Md. But that facility will be only two-thirds the size of the mega-complex in Utah.