In the United States, the copyright for a screenplay generally lasts for the life of the author plus 70 years after their death. If the screenplay was written by a team or group, the copyright lasts for the life of the last surviving author plus 70 years. After the copyright term has expired, the screenplay is considered to be in the public domain and can be used freely by anyone.
It's important to note that a screenplay may be protected by copyright even if it has not been produced or publicly distributed. As soon as the screenplay is fixed in a tangible form (such as being written down on paper or saved on a computer), it is automatically protected by copyright law.
If you are planning to use a screenplay that is still under copyright, you will need to obtain permission from the copyright holder or their representative. You may also need to pay a fee in order to use the screenplay, depending on the terms of the permission granted.