“By working faithfully eight hours a day you may eventually get to be boss and work twelve hours a day” was in the newspaper column “Editorial Epigrams” by Robert Quillen (1887-1948) in 1923. American poet Robert Frost (1874-1963) has been credited with the line since 1940, but there is no evidence that Frost said it before Quillen.
 
   
Wikipedia: Robert Quillen
Verni Robert Quillen (March 25, 1887 - December 9, 1948) was an American journalist and humorist who for more than a quarter century was “one of the leading purveyors of village nostalgia” from his home in Fountain Inn, South Carolina. In 2012, his office and library was listed on the National Register of Historic Places.
     
6 April 1923, The Evening Repository (Canton, OH),“Editorial Epigrams” by Robert Quillen, pg. 4, col. 3:
By working faithfully eight hours a day you may eventually get to be boss and work twelve hours a day.
 
25 February 1928, Bellingham (WA) Herald, “Wheezes and What Not,” pg. 1, col. 1:
Work faithfully for eight hours a day
And don’t worry,
Then in time you may become the boss
And work twelve hours a day
And have all the worry.
 
11 November 1940, Richmond (VA) Times-Dispatch, “On Broadway” by Walter Winchell, pg. 7, col. 4:
R. Frost’s: By working faithfully eight hours a day you can get eventually to be a boss, and work 12 hours a day.
   
2 July 1947, Cleveland (OH) Plain Dealer, pg. 27, col. 5:
‘The Kernel” (cartoon) by Ed Kuekes,
Work eight hours a day with never a care—then success! You become boss—so you work twelve hours a day and are up to your ears in worries.
 
Google Books
The Modern Handbook of Humor
By Ralph Louis Woods
New York, NY: McGraw-Hill
1967
Pg. 7:
By working faithfully eight hours a day, you may eventually get to be a boss and work twelve hours a day. — Robert Frost
     
Google Books
The Concise Columbia Dictionary of Quotations
By Robert Andrews
New York, NY: Columbia University Press
1989
Pg. 326:
By working faithfully eight hours a day, you may eventually get to be a boss and work twelve hours a day.
Robert Frost (1874-1963)
American poet
 
Google Books
The Mammoth Book of Comic Quotes
By Geoff Tibballs
London: Constable & Robinson Ltd.
2004
Pg. ?:
By working faithfully eight hours a day you may eventually get to be a boss and work twelve hours a day.
ROBERT FROST