Figuring out the legal rights necessary to adapt something can seem like an impossible, not to mention expensive, labyrinth for playwrights and screenwriters to navigate. I just chanced upon a helpful document with timetable that sheds light on this, available free online from Cornell University. If you’re thinking of adapting, this could be a useful reference.
The Dramatists Guild of America also provides helpful resources to its members in this area. I assume the Writers Guild offers screenwriters similar help but since I don’t belong, that’s purely a guess.
Given that it can easily take a year or more to complete a script, doing our homework and clearing rights upfront is a wise idea.