How long were world war 1 and 2?

World War I lasted 4 and a half years from April 1914 to November 1918.

World War II was between 4 and 6 years, depending on the starting date. The declared war in Europe was from September 1939 to May 1945. In Asia and the Pacific, there was fighting from December 1941 to September 1945, but Japan had been taking over China beginning in July 1937, and had seized Manchuria as early as 1931.