18 months is about one-third as long as World War I.
In other words, it's 0.3430 times the length of World War I, and the length of World War I is 2.920 times that amount.
(a.k.a. First World War, a.k.a. Great War, a.k.a. World War, a.k.a. War to End All Wars, a.k.a. WWI, a.k.a. WW1) (1914 - 1918) World War I is considered to have begun with the assassination of Austrian Archduke Franz Ferdinand on June 28th, 1914 and to have lasted until the signing of the armistice with Germany on November 11th, 1918, for a total of 52.50 months. However, the Treaty of Versailles which ended conflicts between the major powers was not signed until June 28th, 1919 and the last formal peace treaties were not signed until August 23rd, 1923 — 57.40 months after the armistice was signed.