“Eternity is a ham and two people” (also given as “Eternity is two people and a ham”) is an old quip from the days when a ham was huge—far more than two people could finish. Irma Rombauer mentions this line in her famous cookbook, The Joy of Cooking.
 
New York-based writer Dorothy Parker (1893-1967) is often credited with originating the line, but it is not documented when or if she ever said it. Citations for “ham and two people” appeared in other sources (besides Rombauer) starting from the 1950s.
 
   
Wikipedia: The Joy of Cooking
The Joy of Cooking is one of the United States’ most-published cookbooks, having been in print continuously since 1936 and with more than 18 million copies sold. It was privately published in 1931 by Irma S. Rombauer, a homemaker in St. Louis, Missouri, who was struggling emotionally and financially after her husband’s suicide the previous year. Rombauer had 3,000 copies printed by A.C. Clayton, a company which had printed labels for fancy St. Louis shoe companies and for Listerine, but never a book. In 1936, the book was picked up by a commercial printing house, the Bobbs-Merrill Company. While Joy is considered a bit daunting for beginning cooks, it is the backbone of many home cooks’ libraries and is commonly found in commercial kitchens as well.
     
Wikipedia: Irma S. Rombauer
Irma Starkloff Rombauer (October 1877 - 1962) was the author of The Joy of Cooking. It is one of the world’s most-published cookbooks, having been in print continuously since 1936. She attended Washington University in St. Louis and she privately published the The Joy of Cooking in 1931 in St. Louis, Missouri. It was illustrated by her daughter Marion Rombauer Becker, an art teacher at local private school John Burroughs School. The Rombauers self-published early editions of the book; it was picked up by a commercial printing house, the Bobbs-Merrill Company, in 1936. 
   
Wikipedia: Dorothy Parker
Dorothy Parker (August 22, 1893–June 7, 1967) was an American writer and poet, best known for her caustic wit, wisecracks, and sharp eye for 20th century urban foibles.
 
From a conflicted and unhappy childhood, Parker rose to acclaim, both for her literary output in such venues as The New Yorker and as a founding member of the Algonquin Round Table, a group she would later disdain. Following the breakup of that circle, Parker traveled to Hollywood to pursue screenwriting. Her successes there, including two Academy Award nominations, would eventually be curtailed, as her involvement in left-wing politics would lead to a place on the infamous Hollywood blacklist.
 
Parker survived three marriages (two to the same man) and several suicide attempts, but grew increasingly dependent on alcohol. Although she would come to dismiss her own talents and deplore her reputation as a “wisecracker,” her literary output and her sparkling wit have endured long past her death.
 
27 March 1956, New York (NY) Times, “Smaller Than Ham,” pg. 39:
Eternity has been defined as “a ham and two people.”
 
Google News Archive
20 December 1959, St. Petersburg (FL) Evening Independent, pg. 5D, col. 1:
..and (even though eternity has been described as two people and a ham) ham is such a standard part of our diet and can be stored easily for considerable periods that it avoids most of the leftover stigma.
   
10 August 1960, Long Beach (CA) Press-Telegram, pg. B4, col. 3:
It has been said that eternity is a ham and two people but so is a jar of jam and one.
     
Google Books
Dear Ones All
By Jean Rikhoff
New York, NY: Viking Press
1961
Pg. 240:
Orangy laughed. “My definition of eternity: two people and a whole ham.”
   
27 March 1969, Waterloo (Iowa) Daily Courier, pg. 12, col. 6:
A man once defined eternity as “two people and a ham.” That very bountiful quality of ham, which makes it so beloved by planners of church suppers and big buffets, is apt to deter the small family from having ham more often.
   
4 April 1982, New York (NY) Times, “Ham (Lamb) and an End to Easter’s Leftovers Blues” by Marian Burros, pg. CN14:
“THE definition of eternity,” said one culinary wag, “is a ham and two people.”

Google News Archive
30 June 1986, Bryan (TX) Times, “Eternity is not all nibbles” by Erma Bombeck, pg. 7, col. 1:
Dorothy Parker once described “eternity” as a ham and two people.
 
A close second would be a Thanksgiving turkey and two people or split pea soup and a family of six. But eternity is not all nibbles and leftovers. There are other ways to die slowly.
   
leftover country ham - Chowhound
And yes, flavrmeister, there really is such a thing as leftover ham. The Joy of Cooking quotes “some wag” as having defined eternity as “two people and a ham,” and if you don’t believe it you should come to my house. And while I agree that bean soup at large does not really demand a season, there’s something about hundred-degree weather with high humidity and lots of smog that does not holler, “Hey clark make an enormous batch of pea soup!”
john clark Jun 08, 2003 01:16PM
   
Google Books
American Sandwich: Great Eats from All 50 States
By Becky Mercuri
Salt Lake City, UT: Gibbs Smith
2004
Pg. 128:
Dan is fond of repeating the old Virginia adage, “Eternity has been defined as two people and a country ham,” because frugal folks waste nothing when it comes to such a delicious treat.
 
Gogole Books
Dishing: Great Dish—and Dishes—from America’s Most Beloved Gossip Columnist
By Liz Smith
New York, NY: Simon and Schuster
2005
Pg. xi:
You do know the definition of eternity—two people and a ham.