Once my work is copyrighted, how long is it protected?

Beginning Writer's Answer Book, edited by Jane Friedman

For works copyrighted through the U.S. Copyright Office on or after January 1, 1978, copyright protection lasts for the rest of the author’s life plus seventy years after her death. For works published prior to 1978, the term will vary depending on several factors; see the U.S. Copyright Office Web site (www.copyright.gov) for an explanation.