A work created by an individual is protected for the life of the author plus 70 years. Calculating the length of protection gets more complicated for works for hire or works created anonymously or under a pseudonym. The situation gets even more complicated for works created before 1978 because of significant changes to copyright law in the 1970s. Here is a comprehensive chart on the length of copyright protection: http://www.unc.edu/~unclng/public-d.htm.
How do I know whether something is in the public domain?
Determining whether a work is in the public domain isn’t always easy. The U.S. Copyright Office does not maintain a searchable list of copyrighted works online, but it will investigate the status of a particular work for you at a cost that varies depending on the scope and complexity of the search. Furthermore, the traditional copyright symbol © is not required for something to be copyrighted, so its absence does not mean a work is in the public domain. The value of placing the copyright symbol on your work is that it serves to remind potential users that your work is copyrighted.
If you are in doubt about whether a song, book or other work is in the public domain, it is best to assume that it is not. In the digital age, you can find a story or news photograph on the Internet and copy it with the click of a mouse, but you should not confuse easy access and instantaneous copying with having permission to copy.
 U.S. Copyright Office, How to Investigate the Copyright Status of a Work. Circular 22 (2012), available at http://www.copyright.gov/circs/.