@jespah,
Michael Moorcock is the classic example of a churned. When he was editing New Worlds magazine which was losing money hand over fist, he churned out some of his eternal champion series. Some of the books he wrote in three days.
I have a soft spot for Moorcock, coming at him via Hawkwind in my early teens. In fairness they’re not bad, but he does cheat. The eternal champion is a sword and sorcery constant warrior and he keeps getting reincarnated. Which is quite handy. As each reincarnation has its own set of books and unique sword and sorcery world. The reincarnations are Elric, Corum, Hawkmoon, Erekose, ( and Jerry Cornelius, although he doesn’t appear with the rest.)
On a couple of occasions the reincarnations meet up to fight some cosmic foe. I first started reading the Elric books and came across a crossover for the first time when reading that. When I started reading the Corum books and came across the same scene it had been cut and pasted word for word. Ditto Hawkmoon and Erekose.
Considering how hard he was working and why he can be forgiven.
At the other end of the scale there’s Hubert Selby jr who spent years writing Last Exit to Brooklyn, although his output did pick up after that.