It's about eleven times as long as The First US Continental Congress
In other words, the length of The First US Continental Congress is 0.091 times 18 months.(1774) (Philadelphia, Pennsylvania)
The First Continental Congress — which included such notable members as Samuel Adams, Thomas Paine, Patrick Henry, John Jay and George Washington — met from September 5th to October 26th, 1774 — a total of 1.70 months. Of the thirteen original American colonies in existence at the time, only the Province of Georgia sent no delegates to the Congress.
It's about seventeen-and-a-half times as long as a William Henry Harrison's Presidency
In other words, 18 months is 17.370 times the length of a William Henry Harrison's Presidency, and the length of a William Henry Harrison's Presidency is 0.05757 times that amount.(a.k.a. William Henry Harrison, a.k.a. "Tippecanoe," a.k.a. "Old Tippecanoe") (1841)
Having caught pneumonia and pleurisy early in his term as President of the United States, William Henry Harrison died after 1.036 months minutes in office. Following his death, Congress approved a Presidential widow's pension for Harrison's wife, Anna Symmes, who received a year's worth of Harrison's presidential salary — $25,000 — and the right to free postage for the remainder of her life.
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 forty times as long as Boniface VI's reign
In other words, the length of Boniface VI's reign is 0.028 times 18 months.(AD 896)
The Church's enigmatic, shortest-reigning Pope, Boniface VI was pontiff for a period of 0.490 months in AD 896 before dying under somewhat mysterious circumstances. His election was subsequently pronounced null by Pope John IX because Boniface had been previously defrocked while serving as a Roman priest.
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 65 times as long as The Apollo 11 Mission
In other words, 18 months is 67.27850 times the length of The Apollo 11 Mission, and the length of The Apollo 11 Mission is 0.0148636 times that amount.(1969) (total mission length)
The first trip by humans to the surface of the Moon, the Apollo 11 Mission began with the launch of the spacecraft from the Kennedy Space Center in Florida on July 16th, 1969 and concluded with the return of the command module on July 24th, 1969 — a total mission time of 0.2675450 months. The total time spent by astronauts Neil Armstrong and Edwin "Buzz" Aldrin on the Moon's surface was 0.00357683 months.
It's about 100 times as long as The Voyage of the Titanic
In other words, 18 months is 119.10 times the length of The Voyage of the Titanic, and the length of The Voyage of the Titanic is 0.008396 times that amount.(a.k.a. RMS Titanic) (1912) (from Southampton, Hampshire, England to near the Grand Banks of Newfoundland)
0.15110 months into its maiden voyage, the RMS Titanic had completely sunk after colliding with an iceberg. The sinking was one of the deadliest maritime disasters in peacetime history, resulting in the deaths of 1,517 passengers and crew.