Let's move away from the frustrating and into the amusingly idiotic.
Apparently the longest piece of fiction is a 3.5 million word fanfiction of Smash Brothers, as written by a 21-year-old college junior. It's six times the length of War and Peace. There are writers with conditions like bipolar disorder such as Mark Twain or those who are active users of cocaine who can't muster this level of obsessive motivation to write as much as this guy. And it's all utter trash with all the quality of a fanfiction writer but with the quantity of a printing press.
He's been writing since Brawl's released in...2008, I think. So he's been writing this garbage since he was 11. He also states he hasn't read any novels past his required high school reading, and his main influences are scenarios he acts out while playing the game. That should give you an idea of how engrossing his writing is.
The author himself is god's little verbose gift to the world, where even his profile page is ten thousand words long. According to
this Buzzfeed interview (I know, I know), they sent him 12 incredibly simple questions and his response was over 2,000 words long. It's worth reading because his responses are bewilderingly inane. For example, here's a question regarding if he's ever considered working for a publisher, and somehow he turns the topic towards a Nintendo-themed anime: