It's about two-thirds as long as Pius IX's reign.
In other words, 21 years is 0.66360 times the length of Pius IX's reign, and the length of Pius IX's reign is 1.5070 times that amount.(a.k.a. Pope Blessed Pius IX, a.k.a. Giovanni Maria Mastai-Ferretti) (1792-1878)
The longest reigning Pope in the Church's history, Pius IX reigned from June 16th, 1846 to February 7th, 1878 — 31.650 years. As the last monarchical Pope, he ruled over 3,000,000 subjects in the Papal States until 1870.
It's about one-and-three-fourths times as long as FDR's Presidential Term.
In other words, 21 years is 1.7340 times the length of FDR's Presidential Term, and the length of FDR's Presidential Term is 0.57670 times that amount.(a.k.a. Franklin Roosevelt, a.k.a. Franklin Delano Roosevelt) (1933-45)
FDR, the 32nd President of the United States, served from March 4th, 1933 to April 12th, 1945, dying after 12.110 years in office. At the time of his sudden death, Elizabeth Shoumatoff was painting a portrait of Roosevelt which, never finished, now hangs in a museum at the site of Roosevelt's Presidential retreat in Georgia.
It's about two times as long as King Tut's reign.
In other words, the length of King Tut's reign is 0.50 times 21 years.(a.k.a. Tutankhamun, a.k.a. Tutankhaten, a.k.a. tVwa:t-ʕa:nəx-ʔaˡma:n) (1333 BCE - 1324 BCE)
Pharaoh of the 18th dynasty, Tutankhamun begin his reign at the age of eight in 1333 BCE and remained pharaoh until 1324 BCE — 9 years later. When artifacts from Tut's tomb were on display at the Metropolitan Museum of Art (New York City, New York) from late 1976 to 1979, they were viewed by an estimated 8 million people.
It's about two-and-a-half times as long as The Revolutionary War (American).
In other words, 21 years is 2.5060 times the length of The Revolutionary War (American), and the length of The Revolutionary War (American) is 0.3990 times that amount.(a.k.a. American War of Independence) (1775-83)
The War between Great Britain and the its thirteen colonies in North America (now the United States) for the independence of the United States is generally considered to have started with the Battle of Lexington and Concord on April 18th, 1775 (sometimes known as "The Shot Heard 'round the World") and to have concluded with the signing of the Treaty of Paris on September 3rd, 1783 — 8.3780 years. A total of 25,000 Americans are estimated to have died as a result of the war.
It's about three-and-a-half times as long as The Construction of the Chunnel.
In other words, 21 years is 3.2870 times the length of The Construction of the Chunnel, and the length of The Construction of the Chunnel is 0.30420 times that amount.(a.k.a. Channel Tunnel, a.k.a. Le tunnel sous la Manche) (1987 thru 1994) (Folkstone, Kent, UK to Coquelles, Pas-de-Calais, France)
The construction crew of the Chunnel broke ground in England on December 15th, 1987 and the tunnel was inaugurated by the Queen of England and the President of France on May 6th, 1994 — a total construction time of 6.3820 years. The 4.9 million cubic meters of chalk excavated from the UK side of the Chunnel was dumped within sea walls to create the 299,467.38 sq. km (74 acre) Samphire Hoe County Park near Dover, Kent.
It's about three-and-a-half times as long as World War II.
In other words, 21 years is 3.4970 times the length of World War II, and the length of World War II is 0.2860 times that amount.(a.k.a. Second World War, a.k.a. WWII, a.k.a. WW2) (1939 - 1945)
The most widespread war in history, the Second World War is generally considered to have started with the German invasion of Poland on September 1st, 1939 and to have until the surrender of Japan on September 2nd, 1945, a total of 6.004 years. Also the deadliest conflict in history, the war had an estimated 60 million casualties globally.
It's about five times as long as World War I.
In other words, 21 years is 4.80 times the length of World War I, and the length of World War I is 0.2080 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 4.370 years. 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 — 4.780 years after the armistice was signed.