Generally, for most works created after 1978, protection lasts for the life of the author plus 70 years.
For anonymous works, pseudonymous works, or works made for hire, the copyright term is 95 years from the year of first publication or 120 years from creation, whichever comes first.
When the copyright term expires, a work becomes part of the public domain, and anyone can use it without permission from the author.
Find more information about works that are in the Public Domain at the Lifecycle of Copyright from the U.S. Copyright Office.