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 five times as long as The Spanish-American War
In other words, 18 months is 5.02 times the length of The Spanish-American War, and the length of The Spanish-American War is 0.1990 times that amount.(1898)
The War between Spain and the United States over the liberation of Cuba began on April 25th, 1898 and lasted until the signing of the Treaty of Paris on August 12th, 1898, 3.550 months. The Treaty gave the United States control of Cuba, the Philippines, Puerto Rico, and Guam.
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 six times as long as The First spaceflight (Sputnik)
In other words, the length of The First spaceflight (Sputnik) is 0.170 times 18 months.(a.k.a. Sputnik 1, a.k.a. Спутник-1, a.k.a. "Satellite-1", a.k.a. Простейший Спутник-1) (1957) (total time in orbit)
Launched from Kazakhstan in 1957, Sputnik was the first artificial satellite placed into orbit around Earth, where it remained for 3 months. During its voyage, it traveled about 70,000,000 km (43,495,983.46 mi), completing an orbit of the planet every 0.0022 months.
It's about eight times as long as Columbus' voyage to America
In other words, the length of Columbus' voyage to America is 0.10 times 18 months.(1492) (first voyage)
Attempting to find a western route to Asia, Christopher Columbus set sail on August 3rd, 1492 and landed in the Bahamas on October 12th — 2 months later. After one of Columbus' ships, the Santa Maria ran aground in present-day Haiti, he ordered that the timber be used to used to build a fort and settlement called La Navidad; it remains missing to archaeologists to this day.
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 eight-and-a-half times as long as The Voyage of the Mayflower
In other words, the length of The Voyage of the Mayflower is 0.120 times 18 months.(1620)
Having left Southampton, England on September 16th (new style), 1620, the Mayflower dropped anchor near Cape Cod, Massachusetts 2.20 months later on November 21st (new style), 1620. The voyagers had planned to travel in a convoy of two ships, but the smaller Speedwell was found to be leaking (evidently as a result of sabotage) and all passengers had to be moved to the Mayflower instead.