I recently e-mailed Merriam-Webster's Language Research Service the following question. Associate Editor, Neil S. Serven, replied as follows.
Q: Is it grammatically incorrect to say "a couple hours" instead of "a couple of hours"?
A: Both expressions are standard, though “a couple hours” is considered to be more informal. As such, couple is entered in the dictionary as both a noun and an adjective. The excerpt below from Merriam-Webster’s Concise Dictionary of English Usage discusses the use of couple as an adjective.
While the commentators were worrying whether the noun couple could be used to mean simply "two" and whether it could mean "a few" (see COUPLE, noun), the word itself was following the path of development that dozen had taken centuries earlier—dropping its following of and being used like an adjective. We are not sure when this process began in speech, but we begin to find written evidence in the 1920s. Sinclair Lewis heard it in the dictation of George W. Babbitt:
... all my experience indicates he is all right, means to do business, looked into his financial record which is fine--that sentence seems to be a little balled up, Miss McGoun; make a couple sentences out of it if you have to—Sinclair Lewis, Babbitt, 1922
Lewis was not the only one to use it:
... where the land rises to a couple or three or four feet—W. H. Hudson, Far Away and Long Ago, 1924
... in the phrases a couple peaches, a couple of peaches, only two should be meant—Krapp 1927
G. P. Krapp is the first commentator to mention the construction, but he evidently saw nothing wrong with it. A decade later, however, it was thought to be wrong:
couple. Not an adj.; must be followed by "of" and preceded by article—Muriel B. Carr &} John W. Clark, An A B C of Idiom and Diction, 1937
Of all the subsequent commentators who have disapproved the omission of of, Evans 1957 has the most interesting observation. While insisting that standard English requires of between couple and a following noun, he points out that the of is omitted before a degree word such as more or less. And indeed this construction is found in standard English:
We can end this chapter by looking at a couple more examples of Middle English writing—Charles Barber, The Flux of Language, 1965
... middle-aged men expecting a couple more promotions—Peter Preston, Punch, 28 Nov. 1973
These examples are all British; the construction is explicitly recognized by a recent British dictionary, Longman 1984. The construction occurs in American English too:
... till they had taken a couple more first-class lickings—Elmer Davis, But We Were Born Free, 1954
But American English usage seems to have been influenced by the number of commentators stressing the necessity of of. The result is the occasional " a couple of more":
... a couple of more wins from Jim Palmer—Jim Kaplan, Sports Illustrated, 10 Apr. 1978
Nickles 1974 refers to this construction as a "garble" and opines that it results from confusion of a couple of with some such construction as a few more; he fails to recognize the standard a couple more. Theodore Bernstein seems to have encountered the construction, too; in a June 1967 Winners & Sinners he quotes Evans with a measure of approval, but questions whether all degree words fit the pattern. He comes a cropper by confusing Evans's "degree words" with ordinary adjectives. Bernstein was unable to find any specific comment in usage books on "a couple of more" and concludes therefore that it is not wrong, though "ungraceful." If you find it ungraceful also and do not care to omit the of before more, you can put the more after the noun instead; the example above would become "a couple of wins more from Jim Palmer." Bernstein also notes that when more is promoted to pronoun by omission of the following noun, of is not used, as in "... I think I'll have a couple more."
But we have strayed from the red-blooded, 100-percent-American adjective before a plural noun that Sinclair Lewis heard in the speech of the middle-class Middle West. The usage is apparently not found in British English. Here are a few American ones:
The first couple chapters are pretty good—E. B. White, letter, 26 Oct. 1959
So let's start with a couple samples—Quinn 1980
Afterward, I met Mark Mullaney upstairs for a couple beers—Ahmad Rashad, Sports Illustrated, 25 Oct. 1982
... though Mr. Shaw himself still operated a couple wagons for hire—Garrison Keillor, Lake Wobegon Days, 1985
This construction seems well established in American English. Everyone who comments knows it to be common in speech. It is now quite common in general prose, but we have seldom found it in prose that aspires to formality and elegance. Its two most frequent uses are with periods of time and with number words like dozen, hundred, and thousand:
... have surfaced dramatically in the last couple weeks—James P. Gannon, Wall Street Jour., 16 Oct. 1970
A couple thousand cases of liquor—Wall Street Jour., 14 July 1969
To recapitulate: a couple without of seems to have begun being used like a few and a dozen in the 1920s. It is firmly established in American speech and in general writing (though not the more elevated varieties) when it is used directly before a plural noun or a number word. Before more, a couple is used without of in both British and American English and in this context is often preferred even by American commentators.