 So the duration of a patent is 20 years and the U.S. patent is a monopoly you can use to prevent somebody else from making, using, selling or importing the idea in the United States. This monopoly lasts for 20 years after your filing date and once the 20 years are over cannot prevent anyone from using your idea. After the 20 years, your idea is considered dedicated to the public. That means that anybody else can use your idea without owning any licensing or royalty payments to you or getting your permission, but it also means that nobody else can file a patent on your idea either because it's part of the public domain. The only way they can file another patent is if they improve your idea and come up with some new, novel and non-obvious change.