That's one of the grey areas I didn't want to dive into. With YouTube for example, there's an often quoted time limit of footage, something like 15 seconds, that you can use for fair use, but I don't believe that's actually written anywhere and it just comes down to ratio of video:borrowed footage mixed with common sense. Comes down to the individuals to decide if they want to attempt to sue or cease and desist.

I imagine the same applied to written work, but... yeah it's gritty