How Long Does It Last?
Copyrights do not last forever, but they do last a pretty long time. Under the current laws, copyright protection starts from the moment of creation of the work and continues until fifty years after the death of the author or artist. That means, if someone who is 15 in the year 2005 writes a story that year and dies when he is 85 in the year 2090, the copyright will not expire until fifty years after 2090 -- in the year 2140, which is 135 years away.
What is Public Domain?
When the term of a copyright expires (meaning it's finished), the work falls into what's called the "public domain." When a work is in the public domain anyone can copy it and use it without permission. This is why you can copy artists like Leonardo da Vinci and writers like Shakespeare and music like Mozart all you want. In the case of a joint work prepared by two or more authors, the term of copyright lasts for fifty years after the last surviving author's death.
The terms of many earlier copyrights laws were extended by later laws, and you cannot assume that any work is in the public domain unless it was first published before 1923... Then you're safe... :) See below note...
Note About Copyright Duration
Due to meddling in American politics, we have seen drastic changes over the years in the copyright industry, with many other countries following due to pressure or agreements like CUSMA and NAFTA. See the Mickey Mouse Copyright Act and similar legislation
Expansion of U.S. copyright law (assuming authors create their works at age 35 and die at age 70)