it basically depends on when the creators (all of them ) died, plus a varying number - now about 70 - years.
Google the subject. There are all sorts of sites.
Also please not that extensions can be granted for a number of reasons, so be very careful.
Universal had no rights to "Frankenstein" yet the successfully scared away many adapters with threats of a lawsuit (they came after me for trying to pitch a "Mummy" movie six years before they did the Brendan Fraser versions).
http://www.broadwayworld.com/board/readmessage.cfm?thread=972787#3631451
http://www.broadwayworld.com/board/readmessage.cfm?thread=963561#3533883
http://www.broadwayworld.com/board/readmessage.cfm?thread=955158#3440952
http://www.broadwayworld.com/board/readmessage.cfm?thread=954269#3427915
http://www.broadwayworld.com/board/readmessage.cfm?thread=955012#3441622
http://www.broadwayworld.com/board/readmessage.cfm?thread=954344#3428699