How the heck long does it take for a copyright to expire?
These days it's life of the author plus 70 years, or 100 years depending upon whether or not the work was created by individuals or a corporation "for hire". Got to keep "Steamboat Willie" under copyright forever.
All of Twain's works are public domain now. Most of Mark Twain's works are available for free through Project Gutenberg. I would imagine the copyright issues at question have to do with the play itself, which, if it was written after 1925 or so, would still be under copyright, not the book that it is based upon.
About 70 years. The problem here is that the R&H play based on the novel is a "derivative work" that inovlved a lot of original creative work by the R&H playwrights. Actually, that's the only part that's currently subject to the R&H copyright. However, it's impossible to put on the play without using the copyrighted parts, because they're woven into it just llike fat marblling a roast is woven into the roast. Thus, although the original work is in the public domain and anybody can write and produce a play bassed on it, that same anybody can't legally produce the R&H paly without the R&H permission.