The period of time that a work is protected by copyright depends on when the work was created and/or published. A work created by an individual and first published today in the U.S. would be protected for 70 years after the death of the author. But that’s just the tip of the iceberg; the rules can seem complicated and this chart <pdf> can assist you in determining whether a work is in the public domain, and if not, how long it will be protected by copyright. All works first published in the U.S. before 1923 are in the public domain and are no longer protected by copyright. More at the CUNY Copyright Site.