How many times can one band of misfit heroes save the galaxy? How many times can the fate of the very universe be threatened? These are questions that every fan of science fiction and fantasy eventually must face, because in most speculative fiction series the stakes are always increasing. Defeat a group of local thugs in book 1, battle a literal god in book 6. And that's usually where a series loses me. I can only suspend disbelief so long before I start to wonder why the hero in my fiction hasn't helped to build better processes to protect everyone against what are basically the same repeating threats. Why doesn't my hero ever learn from one story to the next? Are there really that many variables? And so what if there are? Shouldn't my fictional hero be an expert strategist? More importantly, why is Robert E Howard's Conan the Barbarian the one hero immune to this incredulity?
I've been a Conan fan since I discovered what I think were the L. Sprague de Camp paperbacks in my grandfather's workshop. I was a kid then, and I was reading a lot of Star Trek and Star Wars books at the time. Both were starting to wear a little thin for me. With every book, all the heroes basically forgot the events of the previous one (or events from TV episodes and movies) and acted like everything was a completely new experience. As a reader, I'd almost be laughing at the same old strategies being used against the same old threads and leading to the same old outcomes, but the characters in the book would reliably act as if was all new to them. I'd shout at the pages, reminding everybody of the events of Beta Gamma Ypsilon 6 in episode 14 of season 2, but they never listened.
The Conan stories were different. At first it frustrated me, but I couldn't quite pin down Conan's timeline. As a geek, it's basically my job to decipher the background lore of my fiction. It's my responsibility to have at least a hypothesis about when things took place, where in the world, and what those events reveal about the fictional setting. It annoyed me that I couldn't fashion a timeline of Conan's life from the short stories. This made me read more Conan fiction.
I assumed I was missing data. I thought I'd be able to make sense of it all once I had all the data about Conan's life and world. So I read more, and I've been a reader of Conan fiction every since. And I never did make sense of it, which surprisingly did not annoy me.
Recently, it finally occurred to me why Conan was one of the few exceptions to my heroism fatigue. After reading some unsatisfying theories on how Conan stories could fit together into a cohesive timeline, I realised that I found it comforting that Conan's timeline essentially didn't exist. Obviously there's a general sense of his life's journey, from a barbarian warrior of the North with a considerable victory by the age 15, then a rogue, then a pirate, and eventually a literal king. But the more stories you read, the more you get the sense you're reading conflicting reports of Conan's whereabouts both in physical location and in stage of life. I haven't set up the pushpins-and-yarn conspiracy corkboard for this, so I don't have specific examples, but I think it's safe to say that between the books and the comics, there are probably more stories of Conan's campaigns than there would have been years in Conan's adventuring life.
I think it's basically impossible, or extremely unlikely, that all Conan stories are [fictionally] true. And I think that works because most Conan stories do not reference other Conan stories. Keeping in mind that I have no hard data for my claims, I believe that most Conan stories are almost entirely self-contained. You don't need to know anything about Conan, his origin, or his past exploits. Everything you need is in the story you are reading. You can dip into the pool of Conan canon and grab 20 or 30 stories and declare those authentic, and your Conan canon will be equally as defensible as the next reader's.
If your Conan canon is a subset of all stories featuring Conan, then that must mean that the stories you've left on the shelf are either legends or mistakes. I think it's pretty likely that in a world where Conan the Barbarian takes the throne of Aquilonia, stories would start to pop up about his youth. There must be dozens of veteran soldiers who battled among the ranks of a mercenary who'd appeared seemingly out of nowhere to correct the course of their army's campaign. They would have had retrospective cognitive bias about their experiences. Thinking back, that mercenary definitely had a northern, maybe even Cimmerian, look about him. Could his name have been Conan? Yes, it must have been!
And so we have more Conan "stories" than we have Conan history.
Conan stories are rarely about saving reality itself, or preventing planetary annihilation, or even making an impact that has global or regional. It's arguable that his stories are less prone to heroic fatigue because his heroic feats are more relatable than many other fantasy and science fiction quests. Overthrowing a corrupt king or witch queen is impressive, but it makes defeating the same star killer base twice in a row (and the same Emperor, twice) feel a little unlikely.
Conan frequently sought treasure for personal gain or as a mercenary, or rescued someone in trouble, or fought in a regional battle. The stakes are always high but in the grand scheme of the Hyborian age, probably inconsequential. Strictly speaking, Conan can fail or succeed in his quest without changing the course of fictional history. It might set his own priorities back, or it may change a local power structure, but for the most part the rest of the Earth takes no notice of Conan's plight.
To sum it up, I think there are 2 reasons Conan stories are immune to heroic fatigue:
I've actually learned a lot from Conan. As much as I love connecting lore and tracking fictional histories, Conan has taught me that in fiction not everything has to be canon. Take what you like and ignore the stuff you don't. You can't always do this cleanly, because some fiction [sensibly] refers back to previous events in its own timeline. But some fiction doesn't do that, and sometimes that can be a strength. It can be weird, because it feels like your hero's brain gets reset with every new episode or movie or story, but it can also be permission for you to isolate each event in the legend of your fictional hero, and construct your own private canonical timeline.