A question about copyright in the US though, how far back does it reach? 50 years, 100 years?
It's complicated. We're not like most of the rest of the civilized world.
"Works created in or after 1978 are extended copyright protection for a term defined in 17 U.S.C. § 302. With the passage of the Sonny Bono Copyright Term Extension Act, these works are granted copyright protection for a term ending 70 years after the death of the author. If the work was a work for hire (e.g., those created by a corporation) then copyright persists for 120 years after creation or 95 years after publication, whichever is shorter.
Expansion of U.S. copyright termThe § 302 term above also applies to works created before 1978 that were not yet published or registered prior to 1978, with the exception that such copyrights would not expire before 2003. Prior to 1978, works had to be published or registered to receive copyright protection. Upon the effective date of the 1976 Act (January 1, 1978) this requirement was removed and these works received protection despite having not been published or registered. However, Congress intended to provide an incentive for these authors to publish their unpublished works. To provide that incentive, these works, if published before 2003, will not have their protection expire before 2048.
Works published or registered before 1978 currently have a maximum copyright duration of 95 years from the date of publication, if copyright was renewed during the 28th year following publication (such renewal was made automatic by the Copyright Renewal Act of 1992; prior to this the copyright would expire after 28 years if not renewed). The date of death of the author is not a factor in the copyright term of such works.
All copyrightable works published in the United States before 1923 are in the public domain; works created before 1978 but not published until recently may be protected until 2047. For works that received their copyright before 1978, a renewal had to be filed in the work's 28th year with the Library of Congress Copyright Office for its term of protection to be extended. The need for renewal was eliminated by the Copyright Renewal Act of 1992, but works that had already entered the public domain by non-renewal did not regain copyright protection. Therefore, works published before 1964 that were not renewed are in the public domain. With rare exception (such as very old works first published after 2002), no additional copyrights will expire (thus entering the public domain) until at least 2019 due to changes in the applicable laws."