(note this is not legal advice but should be considered a general guide)
General Rule (Safest Assumptions)
All works published in the United States before 1923 are in the public domain. Works published after 1922, but before 1978 are protected for up to 95 years from the date of publication (see Sonny Bono Act of 1998).
For works created after January 1, 1978, copyright protection lasts for the life of the author plus an additional 70 years. For an anonymous work, a pseudonymous work, or a work made for hire, the copyright endures for a term of 95 years from the year of its first publication or a term of 120 years from the year of its creation, whichever expires first.
More Detail on works prior to 1978
For works first published prior to 1978, the term will vary depending on several factors. To determine the length of copyright protection for a particular work, consult chapter 3 of the Copyright Act (title 17 of the United States Code). More information on the term of copyright can be found in Circular 15a, Duration of Copyright, and Circular 1, Copyright Basics.
Do I have to renew my copyright?
No. Works created on or after January 1, 1978, are not subject to renewal registration. As to works published or registered prior to January 1, 1978, renewal registration is optional after 28 years but does provide certain legal advantages. For information on how to file a renewal application as well as the legal benefit for doing so, see Circular 15, Renewal of Copyright, and Circular 15a, Duration of Copyright.
Additional Information for works from 1964 to 1978
Copyright renewal requirements tripped up a lot of copyright owners before the current copyright law did away with them. Failure to renew caused many works originally published from 1923 through 1963 to enter the public domain.
The 1909 Act (the governing law for those works) provided for two copyright terms: a 28-year initial term and a 28-year renewal term. But the copyright owner didn’t get the second term unless he or she filed a renewal application with the Copyright Office. If the renewal application wasn’t filed on time, before the 28th year expired, the work entered the public domain.
Note: When the 1976 Copyright Act was passed, Congress added 19 years to the renewal term, bringing it from 28 years to 47, for a total of 75 years. Copyrights already in their renewal term got the extra 19 years automatically. Copyrights in their initial 28-year term on 12/31/77 got the 47-year renewal term as long as the copyrights were renewed in their 28th year.
The copyrights for many, many works were not renewed. In fact, the US Copyright Office has estimated that less than 15% of works eligible for renewal were, in fact, renewed. That means a lot of works are in the public domain … but it also means you have to find out whether copyright renewal happened, or didn’t.
The years to pay attention to are 1923–1963
Just so we’re clear: when the issue is copyright renewal, you need only worry about works that were published during the years 1923 through 1963. If the work was published before 1923, it’s in the public domain. And a 1992 amendment to the copyright law made renewals automatic for works published from 1964 through 1977 so, as far as renewal goes, you don’t have to worry about them.
Additional reading found here. http://www.publicdomainsherpa.com/copyright-renewal.html
Sonny Bono Act or “Mickey Mouse Protection Act” https://en.wikipedia.org/wiki/Copyright_Term_Extension_Act
Another great source of information. https://fairuse.stanford.edu/overview/public-domain/welcome/