[belorn]

Copyright does not have clear answers. Teachers has always wanted a clear answer to "how many pages can I copy from a book", and they usually get the same answer. There is no cleanly defined number of pages, but fair use do exist so they have to apply they own judgement in each case.

The same questions has been asked about how many seconds of TV/radio is fair to copy, for example as stock footage or in reviews. Many people will say something like "3-5 seconds generally accepted", but each creator has to apply their own judgement per case.

What is a derivative work is one of the most debated aspect of copyright law, likely only second to fair use debates. Only a court of law has any real ability to give a clear answer and then only to a specific case.