How long is a copyright valid? The short answer: Life of the author plus 70 years.
There is a much longer answer. But for most authors, musicians, playwrights, and other creative artists, currently producing creative works, the work is protected by copyright for the life of the creative artist plus 70 years.
(This short answer brings up the need for estate planning by creative artists, but that's a topic for another day.)
For works created after 1977 where corporations or other business entities own the copyright (works for hire), the copyright protection extends for 95 years from the first publication or 120 years from creation, whichever comes first. The same is true for anonymous works or works published under pseudonyms.
The major change in copyright law occurred with the Copyright Act of 1976. Prior to that Act, copyright was valid only for 28 years, subject to one renewal period of 28 years. But as the copyright on some of the major creative works of the 20th Century were poised to expire (think "Mickey Mouse"), Congress was pressured to publishers and movie companies to extend copyright protection.
There are intricacies of the Copyright Act which apply to works created between 1923 and 1977. The protection offered those items depends on when they were created and when they were published. Copyright can vary from 95 years from publication to life plus 70 years, to an absolute deadline of December 31, 2047. If you are dealing with something created prior to 1977, you need to check on the specific applicable law or contact me and I'll try to answer your specific question.
Works published prior to 1923 are in the public domain and do not have copyright protection.*
*Thanks to Kevin Grierson, an intellectual property lawyer in Norfolk, Va., for pointing out the need for a small correction to the original post, which has now been made ("created" in the last paragraph changed to "published"). Kevin also suggested the following link as a good resource for the intricacies of copyright and public domain: http://copyright.cornell.edu/