It's about nine-tenths as long as Pepi II's reign
In other words, 9 decades is 0.9 times the length of Pepi II's reign, and the length of Pepi II's reign is 1 times that amount.(a.k.a. Neferkare) (2278 BCE - 2184 BCE)
The sixth dynasty pharaoh of Egypt's Old Kingdom, Pepi II Neferkare ascended to the throne at the age of six and is generally thought to have reigned until his death at the age of one-hundred — a total of 9 decades. His reign was the longest of any monarch in history.
It's about one-third as long as a Giant Tortoise's Lifespan
In other words, 9 decades is 0.36 times the length of a Giant Tortoise's Lifespan, and the length of a Giant Tortoise's Lifespan is 2.8 times that amount.(for Adwaita, an Aldabra Giant Tortoise) (Dipsochelys dussumieri, formerly Geochelone gigantea)
Adwaita, an Aldabra Giant Tortoise who lived the later part of his life in the Alipore Zoological Gardens in Kolkata, India, was 25 decades at the time of his death. Once a pet of Major-General Robert Clive, 1st Baron Clive, who is credited with securing India as a colony of Great Britain, Adwaita lived through the entirety of India's 19 decades as a British colony.
It's about three times as long as Pius IX's reign
In other words, 9 decades is 2.844 times the length of Pius IX's reign, and the length of Pius IX's reign is 0.3516 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 — 3.1650 decades. As the last monarchical Pope, he ruled over 3,000,000 subjects in the Papal States until 1870.
It's about seven-and-a-half times as long as FDR's Presidential Term
In other words, 9 decades is 7.433 times the length of FDR's Presidential Term, and the length of FDR's Presidential Term is 0.1345 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 1.2110 decades 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 nine times as long as King Tut's reign
In other words, 9 decades is 9 times the length of King Tut's reign, and the length of King Tut's reign is 0.1 times that amount.(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 — 0.90 decades 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 ten-and-a-half times as long as The Revolutionary War (American)
In other words, 9 decades is 10.74 times the length of The Revolutionary War (American), and the length of The Revolutionary War (American) is 0.0931 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 — 0.83780 decades. A total of 25,000 Americans are estimated to have died as a result of the war.
It's about fourteen times as long as The Construction of the Chunnel
In other words, 9 decades is 14.1 times the length of The Construction of the Chunnel, and the length of The Construction of the Chunnel is 0.0710 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 0.63820 decades. 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.