28 CFR 17.24: Duration of classification.
(a) At the time of original classification, original classification authorities shall attempt to establish a specific date or event for declassification not more than 10 years from the date of the original decision based on the duration of the national security sensitivity of the information. If the original classification authority cannot determine an earlier specific date or event for declassification, the information shall be marked for declassification 10 years from the date of the original decision.
(b) At the time of original classification, an original classification authority may exempt specific information from declassification within 10 years in accordance with section 1.6(d) of Executive Order 12958.
(c) An original classification authority may extend the duration of classification or reclassify specific information for successive periods not to exceed 10 years at a time if such action is consistent with the standards and procedures established under, and subject to the limitations of, Executive Order 12958.