Yeah, patents only last 20 years from the date of the application. In the 1950s, it was 17 years from the date the patent was granted, which usually turned out to be more like 25-30 years total. Back in the day, Polaroid maintained their near monopoly by constantly improving the product and patenting the improvements, like the technology required to go from black-and-white to color, or not having to wipe the photo down with a sort of "fixing" brush.
This is unlike copyright, which has to be "for a limited time" according to the U.S. Constitution, but thanks to Mickey Mouse that "limited time" appears to be Forever minus one day, with Disney going to Congress to get the copyright extended every time "Steamboat Willie," and therefore Mickey Mouse, is about to fall into the Public Domain. At the time the U.S. Constitution was written, copyright was for 14 years. In the first part of the 20th Century, copyright was 28 years, renewable for another 28. The comic book character "Richie Rich," for example, fell into the public domain when the author/publisher failed to renew.
But when in the 1970s the copyright on "Steamboat Willie" was about to expire, Disney and other Big Media went to Congress crying about "aging authors outliving their copyrights and starving to death because they could no longer charge royalties for them." Congress bought it (more precisely, they bought Congress) and copyright was extended. It happened again in the 1990s, where some works already in the Public Domain were put back into copyright, and today copyright is for the lifetime of the author plus 76 years, or 96 years for a "work for hire," where the copyright belongs to the author's employer rather than the author.
I just wish MY great-great-grandchildren could collect payments for the work I spent MY life doing! But for Disney and the big publishing houses, it's the "works for hire" clause that is of interest because in many cases they have authors perjure themselves by saying what they wrote was a "work for hire" so the publisher gets the royalties, not the author. One author, Orson Scott Card, has actually suggested in print (or on a blog, which amounts to the same thing these days) that copyright be limited to the longer of lifetime of the author plus 20 years or 100 years, while copyright on works for hire should be limited to 20 years, so publishers would be forced to build good relationships with their authors!
I don't know where this is all going, but based on the forever minus one day mentality, I believe some time in the early 2020s, we're going to see copyright extended again....