Originally Posted by: RazboIs there a general period of time? I tried wiki-ing it, and came up with 50-100 years after the author's death.

From this site:

http://www.pdinfo.com/copyrt.php

Works created after 1/1/1978 - life of the longest surviving author plus 70 years - earliest possible PD date is 1/1/2048

Works registered before 1/1/1978 - 95 years from the date copyright was secured.

Works registered before 1/1/1923 - Copyright protection for 75 years has expired and these works are in the public domain.

You should use a public domain composition only if you have proof of public domain from a legitimate source.

And the ultimate legit source is of course the Library Of Congress, holder of copyright claims.