While not a Copyright expert, C.G.P. Grey has provided an interesting and entertaining look at the length of copyright protection in this short video. By the way, while I like the video, one error which stood out to me is that the length of copyright in 1790 in the United States was 14 years, with the possibility of renewal for another 14 years.  Yes, that does equal 28, which is what he said in the video, but only 14 years was guaranteed with additional action by the creator.