The duration of copyright is typically 70 years after the life of the author of the work. Given that the work was created by one author.There are always exceptions with this especially when it comes to corporate ownership. Corporations are able to keep the duration of copyright for 95 years after the publication date.
I once heard that Disney does a new release on old videos such as "The Little Mermaid" and then puts it back in the "vault" so to speak in order to keep the copyright on that material. The new release may be digitally enhanced or new material added to the old copy in order to make it new. Personally, I think that works like that that are obvious that Disney is the owner of the copyright should be able to keep that copyright till the end of the corporation. So in the case of Disney, all of their works would be copyright protected for 95 years after the business in no longer running.