Copyright duration refers to how long copyright remains valid before a work enters into the public domain and can be freely used by anyone without attribution.
Any work created before 1923 is automatically in the public domain.
Any work published between 1923 and 1977 but published without a copyright notice is in the public domain.
Any work published between 1978 and 1 March 1, 1989 without a copyright notice and without subsequent registration with the Copyright Office is in the public domain.
Cornell's Chart for Copyright duration