Traditionally, a bank’s hours have been short, such as from 10 a.m. to 3 p.m. The hours have almost never extended to the late afternoon or early evening, or on the weekends. To work “bankers’ hours” means to work a very reasonable amount of time, with plenty of time left over for other activities.
   
The term “bankers’ hours” has been cited in print since at least the 1920s. The old “3-6-3 Rule” was based on giving 3% interest on accounts, charging 6% interest on loans, and the banker being on the golf course by 3 p.m. The term “bankers’ hours” has been less popularly used since the rise of 24-hour online Internet banking.
   
 
Wiktionary: bankers’ hours
Etymology
From the traditional opening hours of banks in the 1800s.
Noun
bankers’ hours (uncountable)
1.The period between 10am and 3pm.
2.(figuratively) Any easy job, especially one with a short working day.
 
The Free Dictionary
bank·ers’ hours (bngkrz)
pl.n.
A short working day.
 
Dictionary.com
bankers’ hours
–plural noun
a short working day, especially one that extends from 9 or 10 a.m. until 2 p.m., the hours during which banks were traditionally open to the public.
 
Google Books
March 1920, The Railroad Telegrapher, pg. 384, col. 1:
Bro. Smith Is working bankers’ hours — 10 a. m. to 6 p. m.
 
10 September 1925, Lima (OH) News, pg. 12, col. 2 classified ad:
YOU WANT TO
MAKE MONEY
AND
You can make it
selling our
product.
Seventy-five percent of calls are sales. Exclusive territory for the right party. One of our representatives made sixteen hundred dollars in one week. You can work bankers’ hours and make fifteen to fifty dollars a day.
 
21 June 1944, Oakland (CA) Tribune, “Polly Pippen” comic, pg. 11, col. 1:
Panel 1:
HUSBAND: I’LL HAVE THIS ALARM CLOCK TOGETHER AND BETTER THAN NEW IN A JIFFY!
Panel 2:
HUSBAND: IT’S FIXED, POLLY! I’LL SET IT FOR TEN!
Panel 3:
WIFE: TEN? BUT YOU HAVE TO BE UP BY SEVEN!
Panel 4:
HUSBAND: I KNOw, POLLY, BUT I FIXED IT SO THAT WHEN THE ALARM GOES OFF AT SEVEN, THE CLOCK SAYS TEN—IT’LL LOOK LIKE I’M WORKING BANKER’S HOURS!
   
Google Books
A Guide to Archaeological Field Methods
By Robert F. Heizer
Palo Alto, CA: National Press
1958
Pg. 153:
Archaeologists do not work bankers’ hours.
         
Google Books
General Business for Everyday Living
By Raymond Glen Price, Vernon Armor Musselman and Edwin E. Weeks
New York, NY: Gregg Pub. Division, McGraw-Hill
1960
Pg. 79:
Have you ever heard anyone say, “It must be wonderful to work bankers’ hours”? From one of your local banks, find out just what the working hours are for bank employees. How do these hours compare with the working hours for people in other occupations in your community? How many hours do the bank employees work after the bank is closed for the day? Do the employees work on Saturday?
 
Google Books
Types of Medical Practice:
Making your choice
By Norton Mockridge
Oradell, NJ: Medical Economics Books
1982
Pg. 42:
One wrong way to attract your colleagues’ attention is to get a reputation for working “bankers’ hours.” A much better way to win their support is to let it be known that you’ll take emergency work whenever it turns up, without any fussing or fuming about specialty lines, and then do it.
 
Bankrate.com
So, how many hours do bankers work?
By Jay MacDonald • Bankrate.com
Just what are bankers’ hours? The most accurate answer is “elusive.”
 
The Oxford English Dictionary doesn’t list it. Neither does Webster’s.
 
The Dictionary of American English has it but hedges its bet, defining the phrase as: “The hours during which a bank is open to transact business.” Duh.
 
The American Heritage Dictionary of English Usage offers a blissfully succinct definition: “A short working day.”
(...)
But as any traveler knows, the true definition of bankers’ hours is 15 minutes shorter than you thought they were.
Jay MacDonald is a contributing editor based in Florida.
—Posted: May 8, 2002
 
Google Books
Red Rabbit
By Tom Clancy
New York, NY: Berkley2003
Pg. ?:
“Must be nice to work bankers’ hours.”