Every federal agency has a records retention schedule with stipulates how long certain records are kept by the agency. These are useful to know how long records are kept, and what may, or may not, be available.
Below you will find the relevant records obtained from the agencies for reference purposes.
Defense Information Systems Agency (DISA) Records Retention Schedule [198 Pages, 1mb]
Federal Bureau of Investigation (FBI) File Number 66-HQ-3286 - Destruction of Bureau Records 1944-1996 [7,426 Pages, 459MB] (Source: GovernmentAttic.org)
National Reconnaissance Office Records Control Manual (1995) [202 Pages, 20.57mb]
National Reconnaissance Office Review and Redaction Guide for Automatic Declassification of 25-Year Old Information [274 Pages, 34.25mb]
FBI - Records Management Overview Training Video [37 Minutes, 30 Seconds, 581.1mb]