A motion to adjourn is almost always in order, with an exception being when a vote is in progress. The motion is not debatable and is not subject to amendment. “The motion to adjourn is always in order” has been cited in print since at least 1840.
 
Science fiction author Robert Heinlein (1907-1988) popularized “A motion to adjourn is always in order” as a proverb in Time Enough for Love (1973). The suggestion of “a motion to adjourn is always in order” is a bit of legislative humor, implying that it might be time for everyone to stop legislating and to go home.
 
 
27 June 1840, New-York (NY) Commercial Advertiser, “Legislature of New-York,” pg. 2, col. 3:
And the motion to adjourn is always in order.
 
22 June 1852, New-York (NY) Times, “Whig National Convention,” pg. 1, col. 5:
Mr. Cabell raised a point of order, that a motion to adjourn is always in order, and the resolution was not in order, unless the Convention shall rescind or repeal the rule.
 
25 April 1868, The Daily Citizen (Jackson, MI), pg. 4, col. 1:
THE WEATHER—A weather item, like a motion to adjourn, is always in order.
 
Google News Archive
23 February 1871, Western Home Journal (Lawrence, KS), pg. 2, col. 6:
The Washington correspondent of the Cincinnati Commercial reports this rich Congressional scene:
 
Judge Winans, who is ordinarily grave as a gravestone, kept the House laughing yesterday, while attempting to get the Speaker’s eye, and so obtain the floor.
(...)
Winans—I rise to a point of order.
Speaker—The gentleman from Ohio will state his point.
Winans—That a motion to adjourn is always in order. [Great laughter.]
 
Google News Archive
20 August 1879, The Daily Telegraph (Quebec), “Humorous,” pg. 4, col. 2:
The Keokuk Constitution suggests to the Weather Clerk that a motion to adjourn is always in order.
 
Google Books
Time Enough for Love
By Robert A. Heinlein
New York, NY: G. P. Putnam’s Sons
1973
Pg. ?:
A motion to adjourn is always in order.
   
New York (NY) Times
REAGAN, THEIR TARGET, FACES HIS CARTOONISTS
By WARREN WEAVER Jr., Special to the New York Times
Published: May 08, 1987
(...)
At the end of the meeting, an unidentified cartoonist bailed the President out. Reporters had begun shouting questions about the Iran hearings from the back of the audience. Mr. Reagan answered one and was edging for the door when a guest yelled, ‘‘I move we adjourn.’‘
 
Mr. Reagan replied gratefully, ‘‘A motion to adjourn is always in order,’’ and retreated into the White House.
 
Google Books
Murphy’s Law:
The 26th Anniversary Edition
By Arthur Bloch
New York, NY: Perigee
2003
Pg. 58:
Parker’s Rule of Parliamentary Procedure
A motion to adjourn is always in order.
   
LewRockwell.com
June 17, 2006
Civil Society’s Rules of Order
by Gil Guillory
(...)
In an interminable meeting you may have heard someone quip, “a motion to adjourn is always in order”. That’s almost true: the only pending business that a motion to adjourn cannot interrupt is the consideration of a motion to set a specific time of adjournment.
 
Google Books
Samuel R. Wells’s How to Behave:
The Classic Pocket Manual of Good Manners & Model Behavior
By Samuel R. Wells
Avon, MA: Adams Media’
2013
Pg. 159:
PRIVILEGED QUESTIONS
There are certain motions that are allowed to supersede a question already under debate. These are called privileged questions. The following are the usually recognized privileged questions:

1. Adjournment.—A motion to adjourn is always in order, and takes precedence of all others; but it must not be entertained while a member is speaking, unless he give way for that purpose, nor while a vote is in progress. it is not debatable, and cannot be amended.