It's about one-one-hundred-fiftieth as long as a Giant Tortoise's Lifespan.
In other words, the length of a Giant Tortoise's Lifespan is 170 times 18 months.(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 3,000 months 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 2,300 months as a British colony.
It's about one-fiftieth as long as Pepi II's reign.
In other words, the length of Pepi II's reign is 50 times 18 months.(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 1,000 months. His reign was the longest of any monarch in history.
It's about one-twentieth as long as Pius IX's reign.
In other words, 18 months is 0.04737 times the length of Pius IX's reign, and the length of Pius IX's reign is 21.110 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 — 380 months. As the last monarchical Pope, he ruled over 3,000,000 subjects in the Papal States until 1870.
It's about one-tenth as long as FDR's Presidential Term.
In other words, 18 months is 0.12380 times the length of FDR's Presidential Term, and the length of FDR's Presidential Term is 8.078 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 145.40 months 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 one-fifth as long as The Revolutionary War (American).
In other words, 18 months is 0.17890 times the length of The Revolutionary War (American), and the length of The Revolutionary War (American) is 5.590 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 — 100.60 months. A total of 25,000 Americans are estimated to have died as a result of the war.
It's about one-fifth as long as King Tut's reign.
In other words, the length of King Tut's reign is 5 times 18 months.(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 — 100 months 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 one-fourth as long as The Construction of the Chunnel.
In other words, 18 months is 0.23460 times the length of The Construction of the Chunnel, and the length of The Construction of the Chunnel is 4.2630 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 76.630 months. 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.