The length of copyright is normally a function of  the date of publication, the date of creation and the death of the author. In general:

• Copyright expires 70 years after the death of the author or, in the case of joint works, the last surviving author.
• In the case of corporate authorship, copyright expires 95 years after the publication date.
• In the case of unpublished works where the author’s identity or date of death is not known, copyright expires 120 years after the work’s creation.

There are complicating factors, and identifying whether material is protected or in public domain can be complex. For help, see Cornell’s Copyright Term and the Public Domain table.

The law governing copyright term is found in 17 USC Ch. 3.