From the time of its start until the approach of World War II, the First World War was called simply the World War or the Great War and thereafter the First World War or World War I. At the time, it was also sometimes called "the war to end war" or "the war to end all wars" due to its then-unparalleled scale and devastation. In Canada, Maclean's magazine in October 1914 wrote, "Some wars name themselves. This is the Great War." During the interwar period , the war was most often called the World War and the Great War in English-speaking countries. The term "First World War" was first used in September 1914 by the German biologist and philosopher Ernst Haeckel, who claimed that "there is no doubt that the course and character of the feared 'European War' ... will become the first world war in the full sense of the word," citing a wire service report in The Indianapolis Star on 20 September 1914. After the onset of the Second World War in 1939, the terms World War I or the First World War became standard, with British and Canadian historians favouring the First World War, and Americans World War I. In the introduction to his book, Waterloo in 100 Objects, historian Gareth Glover states: "This opening statement will cause some bewilderment to many who have grown up with the appellation of the Great War firmly applied to the 1914-18 First World War. But to anyone living before 1918, the title of the Great War was applied to the Revolutionary and Napoleonic wars in which Britain fought France almost continuously for twenty-two years from 1793 to 1815." In 1911, the historian John Holland Rose published a book titled William Pitt and the Great War.
Question: How many years after World War I began did the terms the First World War and World War I become standard?
Answer: 25