"A couple without of 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." (Merriam-Webster 302)
"The type 'a couple weeks later'. i.e. with no of between a couple and weeks, is well established in AmE."
"A couple of = about two is used informally (I'll just tbe a couple of minutes = in approximately two minutes). (Burchfield 187)
Merriam-Webster's Dictionary of English Usage. Reprint ed., Merriam-Webster, 1994.
Fowler's Modern English Usage. R. W. Burchfield and H. W. Fowler, revised 3rd ed., Oxford UP, 2004.