American humorist Erma Bombeck (1927-1996) wrote in her syndicated newspaper column in November 1988 about Thanksgiving dinner:
 
“Do men ever really appreciate what women have to go through to pull off a traditional Thanksgiving dinner? (...) The average time for eating a Thanksgiving dinner is 12 minutes, which, incidentally, coincides with halftime. The guests will now observe, ‘I’m stuffed! I can’t eat another bite. Forget the pie until my dinner settles.’”
 
By at least 1995, Bombeck’s words changed to this popular form:
 
“Thanksgiving dinners take eighteen hours to prepare. They are consumed in twelve minutes. Halftimes take twelve minutes. This is not a coincidence.”
 
“On Thanksgiving Day all over America, families sit down to dinner at the same moment — halftime” is a similar saying.
   
   
Wikipedia: Erma Bombeck
Erma Louise Bombeck (née Fiste; February 21, 1927 – April 22, 1996) was an American humorist who achieved great popularity for her newspaper column that described suburban home life from the mid-1960s until the late 1990s. Bombeck also published 15 books, most of which became bestsellers. From 1965 to 1996, Erma Bombeck wrote over 4,000 newspaper columns, using broad and sometimes eloquent humor, chronicling the ordinary life of a midwestern suburban housewife. By the 1970s, her columns were read twice-weekly by 30 million readers of the 900 newspapers in the U.S. and Canada.
 
24 November 1988, Palm Beach (FL) Post, “halftime regulates Thanksgiving dinner” by Erma Bombeck, pg. 20D, col. 1:
Do men ever really appreciate what women have to go through to pull off a traditional Thanksgiving dinner?
(...) (Col. 2—ed.)
The average time for eating a Thanksgiving dinner is 12 minutes, which, incidentally, coincides with halftime. The guests will now observe, “I’m stuffed! I can’t eat another bite. Forget the pie until my dinner settles.”
     
Google Groups: eunet.jokes
E-Mail Humor - A Look WAY Back: 11/06/95
Jim Moore Jr
11/6/98
Ever wonder what jokes were being passed around three years ago today.  Below are a few from the UGA Humor List Archives (see URL below) from the Nov 6 1995 Digest, submitted by various contributors all over the world:
(...)
Thanksgiving dinners take eighteen hours to prepare. They are consumed in twelve minutes.
 
Halftimes take twelve minutes.  This is not a coincidence.
   
Google Groups: alt.quotations
Happy Thanksgiving; and no leftovers, please
Grace McGarvie
11/28/02
THANKSGIVING
Thanksgiving dinners take eighteen hours to prepare.  They are consumed in twelve minutes.  This is not a coincidence.  Erma Bombeck
 
Google Books
America in Quotations
Edited by Bahman Dehgan
Jefferson. NC: McFarland & Company, Inc.
2003
Pg. 83:
Thanksgiving dinners take eighteen hours to prepare. They are consumed in twelve minutes. Half-times take twelve minutes. This is not coincidence.—Erma Bombeck, ¡927–
 
Facebook
Wishing you a safe & warm Thanksgiving!
November 24, 2010 at 12:00pm
(...)
Thanksgiving dinners take eighteen hours to prepare. They are consumed in twelve minutes. Half-times take twelve minutes. This is not coincidence.
Erma Bombeck
 
16 November 2011, The Times (Shreveport, LA), “Thanksgiving traditions never go out of style” by Marlyn Monette:
The late Erma Bombeck, popular American humorist, once made the observation that a Thanksgiving dinner takes about 18 hours to prepare and is consumed in 12 minutes. Football half-time takes 12 minutes. This is not a coincidence.
 
23 November 2015, Argus Leader (Sioux Falls, SD), “Thanksgiving stories: fun, memories” by Shirley Halleen, pg. A6:
Erma Bombeck was once quoted as saying that Thanksgiving dinner can take 18 hours to prepare and it is consumed in about 12 minutes. Half-time takes about 12 minutes. This is not a coincidence.