I would point you to the Department of Defense's declassification guidance. This is a ponderous tome, but you can find the following on page 40.

tl;dr 10-25 years is a typical "declassify on" period, 50-75 years can happen in cases involving WMD or human intelligence sources.

OCA = Original Classification Authority

ISCAP = Interagency Security Classification Appeals Panel

a. Originally Classified Information. At the time an item of nformation is classified, the OCA shall establish a specific date or event for declassification, based on the duration of the national security sensitivity of the information. The OCA shall use one of the following duration options, selecting, whenever possible, the one that will result in the shortest duration of classification.

(1) A date or independently verifiable event less than 10 years from the date of the document;

(2) A date 10 years from the date of the document;

(3) A date or independently verifiable event greater than 10 and less than 25 years fromthe date of the document;

(4) A date 25 years from the date of the document;

(5) “50X1-HUM,” designating a duration of up to 75 years, when classifying information that could clearly and demonstrably be expected to reveal the identity of a confidential human source or a human intelligence source;

(6) “50X2-WMD,” designating a duration of up to 75 years, when classifying information that could clearly and demonstrably be expected to reveal key design concepts of weapons of mass destruction; or

(7) “25X” with date or event, designating a duration of up to 50 years when classifying information that clearly falls within an exemption from automatic declassification at 25 years that has previously been approved by the ISCAP.