Answering the question about how long a copyright lasts, or whether a particular copyrighted work is in the public domain and can be readily used without the payment of royalties, is an impossible question to answer in the abstract.  This is because over the years the United States Congress has periodically altered the length of copyright protection and the formalities that must be followed in order to establish a copyright that can be enforced against others.  What this has done is create a bit of a puzzle that requires one to know when the work was created, and in some instances when the work was published.

So what can be said with confidence?  Here goes…

Works Originally Created on or after January 1, 1978

A work that is created on or after January 1, 1978, is automatically protected from the moment of its creation and is ordinarily given a term enduring for the author’s life plus an additional 70 years after the author’s death. In the case of “a joint work prepared by two or more authors who did not work for hire,” the term lasts for 70 years after the last surviving author’s death. For works made for hire, and for anonymous and pseudonymous works, the duration of copyright will be 95 years from publication or 120 years from creation, whichever is shorter.

Works Originally Created before January 1, 1978, But Not Published or Registered by That Date

These works have been automatically brought under the current statute and are now given federal copyright protection. The duration of copyright in these works will generally be computed in the same way as for works created on or after January 1, 1978: the life-plus-70 or 95/120-year terms will apply to them as well. The law provides that in no case will the term of copyright for works in this category expire before December 31, 2002, and for works published on or before December 31, 2002, the term of copyright will not expire before December 31, 2047.

Works Originally Created and Published or Registered before January 1, 1978

Under the law in effect before 1978, copyright was secured either on the date a work was published with a copyright notice or on the date of registration if the work was registered in unpublished form. In either case, the copyright endured for a first term of 28 years from the date it was secured. During the last year of the first term, which is the 28th year of protection, the copyright was eligible for renewal. The Copyright Act of 1976 extended the renewal term from 28 to 47 years for copyrights that were subsisting on January 1, 1978, or for pre-1978 copyrights restored under the Uruguay Round Agreements Act (URAA), making these works eligible for a total term of protection of 75 years. Public Law 105-298, enacted on October 27, 1998, further extended the renewal term of copyrights still subsisting on that date by an additional 20 years, providing for a renewal term of 67 years and a total term of protection of 95 years.

Even more confusing, but worth knowing

Public Law 102-307, enacted on June 26, 1992, amended the 1976 Copyright Act to provide for automatic renewal of the term of copyrights secured between January 1, 1964, and December 31, 1977. Although the renewal term is automatically provided, the Copyright Office does not issue a renewal certificate for these works unless a renewal application and fee are received and registered in the Copyright Office. Public Law 102-307 makes renewal registration optional. Thus, filing for renewal registration is no longer required in order to extend the original 28-year copyright term to the full 95 years. However, some benefits accrue from making a renewal registration during the 28th year of the original term.

For more information see Copyright Term and the Public Domain in the United States.