How many years did it take to ultimately settle the frontier?

The American frontier was considered settled by the end of the 19th century, particularly around 1890 when the U.S. Census declared the frontier closed. The process of settlement began in earnest with westward expansion in the early 1800s, particularly after the Louisiana Purchase in 1803 and the subsequent waves of migration. Thus, it took approximately 90 years of significant settlement efforts to fully develop and integrate the western territories into the United States.