When does something lose its copyright and become available to use under public domain? 

Peter Hirtle, Archivist at Cornell University, has updated his very useful chart. Frequently updated.  Anything that was published before 1923 is in the public domain. In addition, anything published between 1923 and 1977 without a copyright notice is in the public domain.

Lolly Gasaway's chart is a helpful shorter chart.