It took Stephen King 4 decades to complete the Dark Tower series. Glad I started them later so that I didn't have that much of a wait, but, they were worth whatever wait it took.
the Dark Tower books
Main article: The Dark Tower (series)
In the late 1970s, King began what became a series of interconnected stories about a lone gunslinger, Roland, who pursues the "Man in Black" in an alternate-reality universe that is a cross between J. R. R. Tolkien's Middle-earth and the American Wild West as depicted by Clint Eastwood and Sergio Leone in their spaghetti Westerns. The first of these stories, The Dark Tower: The Gunslinger, was initially published in five installments by The Magazine of Fantasy & Science Fiction under the editorship of Edward L. Ferman, from 1977 to 1981. The Gunslinger was continued as an eight-book epic series called The Dark Tower, whose books King wrote and published infrequently over four decades.