First let me say that there are no hard-and-fast rules in storytelling. As with any aspect of craft, choices about how to do chapters are driven by the experience we want our readers to have. It's about the effects our choices have on how readers understand and experience our stories. Decisions about chapter length and breaks are mostly about pacing. Since I like reading fast-paced page-turners, that's the effect that I go for in my own writing. I want my books to be unputdownable, so that's my mindset when I'm making decisions about chapters.
There's no minimum. No, really. I've seen a one-word chapter done successfully before. (See also these examples, which include several chapters that one-up the one-word chapter by containing zero words.) That said, if I'm reading a book that has a bunch of super-short chapters, each one is going to have to be riveting and compelling to prevent me from getting frustrated or fatigued.
But look at James Patterson's 7th Heaven. Those chapters are, on average, 3-5 pages (approx. 900-1500 words) long. That's it. The short chapters contribute to that fast-paced feel. They make it easy for me to say, "Just one more chapter. Well...maybe just one more. Okay, this is the last one. Or maybe the next one." Next thing I know, it's 4am and I'm finishing the book.
So there's no minimum, but I would say that there's a maximum. Ideal chapter length might vary depending on audience and genre. One would expect chapters in a MG book to be consistently short. A literary novel can get away with having what I consider to be pretty darn long chapters - 20-25 pages (approx. 6K-7500 words) - as can an epic fantasy or a long scifi novel. Romance? Not so much. I'm writing genre fiction, so I like to keep mine no longer than 10 pages (approx. 3000 words).
If you want to get a sense of how long chapters tend to be in books for your target audience (MG, YA, NA, A) and in your genre, go pick up five books in your audience/genre (bestsellers would be great because they're obviously doing it right) by five different authors (very important!) and start counting pages. Note whether there's a trend toward shorter chapters at the beginning that gradually lead into longer chapters. Note whether the chapter length is relatively consistent in Book X or whether the chapter lengths vary greatly.
If you don't care so much about what's usual for your audience and genre but instead are more interested in creating the experience you like as a reader, you can perform the exercises above with books you admire.
It seems like common sense to end a chapter at the end of a scene. And sometimes that is entirely appropriate. But you know those page-turning novels that suck you in and don't let you go until the end (or at least the end of your lunch break)? Chances are excellent that their chapters don't always end when a scene does.
Again, this is about reader experience. I want to create a riptide with my words that pulls readers into my stories and only lets them up to breathe on rare occasion, if at all. Chapter breaks are super important to creating this effect; I want every chapter to end in such a way that my reader feels compelled to start the next chapter right away: they have to know what's going to happen next!
If the chapter break coincides with the end of a scene, what pulls us into the next chapter? Often it's a question we want the answer to or the promise of a scene we're eager to read.
At the end of Chapter 23, Detective Grover discovers a lipstick stain on a coffee mug at the crime scene. It's an unusual color lipstick that the detective saw the neighbor, Mrs. Howell, wearing, but she said she wasn't at the crime scene. Detective Grover leaves the crime scene. End scene, chapter. We keep reading because we expect the next scene to be a confrontation between the detective and the neighbor, and we want to know how she's going to explain herself.
But there are also effective ways to break up scenes. My favorite is to drop a bomb right before the chapter break.
In Chapter 12, our young heroine is hiding from the villain, just trying to get out of there because she isn't equipped at this moment to fight The Big Fight. The tension builds, the heroine's heart pounds, she sees her opportunity to get out, and...the villain catches her! End chapter. (Dun, dun, dunnnnn.) In a broken-up scene, the next chapter would pick up exactly where the previous one ended (no skipping time, no change in location), and we keep reading because The Thing We Didn't Want to Happen has happened and we have to know how she's going to get out of it.
A slight variation on the bomb-drop chapter break can be used in a book told from multiple POVs. You're reading along in Character A's POV, and Character A drops a bomb. End chapter! Then the next chapter switches to Character B's POV to see how they handle that bomb.
End of Ch. 17: "I have something to tell you." He closed his eyes, took a deep breath, and let it out slowly. How had he let it get this far? She was so going to hate him. "My name's not Dwayne, and I'm not from Alabama. My name is actually...well, something the human mouth can't pronounce. And I'm from another planet."
Beginning of Ch. 18: Terra blinked. And blinked again. Waiting for Dwayne to break out that big smile that made her heart flutter, for him to laugh, to assure her it was all a joke. But he didn't. Which left her with only one logical conclusion. Her heart sank into her stomach and a sour lump formed in her throat. Dwayne Cooley was certifiably insane.
To Title or Not to Title?
I have so much admiration for people who can do chapter names well. Coming up with great titles is super hard for me, and that's in essence what chapter names are: titles. When done right, they can add to the reader's experience by provoking the reader's curiosity and/or providing a new layer of meaning to the events in that chapter.
Because titles don't come easily to me, I just stick with numbering my chapters. But if you have chapter titles or are considering adding them, I'd encourage you to ask yourself: What purpose(s) do these chapter titles serve? Who are they for? What value do they add to the reader's experience? (And if you're not sure, ask your beta readers.)
One might argue that chapter titles are at worst innocuous. But I disagree. When I come across chapter titles that don't add something meaningful to the chapter, I first spend time trying to understand why the author included them (because why include them if they weren't important??), and then I start skipping them. Then I have to remember to skip that text at the beginning of each chapter. Maybe it only takes an extra half-second at the beginning of the chapter, but it's a dream-breaker. It interrupts my reading experience. I become annoyed. Maybe not annoyed enough to stop reading, but why do you want to do this to me? Therefore I argue: Don't put any words in your ms that don't need to be there. This includes chapter titles.
Indicating POV switches
There is one other kind of text that sometimes appears between the chapter heading and the beginning of the chapter: a name to indicate a POV switch. This happens in novels that are told from multiple POVs and is a device intended to help the reader follow POV switches. This is really only necessary when both/all POVs are told in first-person and the characters' voices aren't immediately distinguishable within the first sentence or two.
Related Questions from Twitter
Q: In dual POV, should chapter length and number of chapters from each POV be equal? Or can one MC who has a bigger character arc get more attention?
A: If the book is about both characters equally, then their POVs should get roughly equal page time. But if a secondary character's POV is necessary to include from time to time (or even just once) because that's the best way to tell the story, then no, you don't have to give them equal page time. (You'd have to argue real smart, though, to convince me that the other person's POV is necessary.)
Q: How big a bomb does it need to be to force an early chapter break?
A: I'm not sure that I would argue that the bomb has to be a certain bigness; rather, I would argue that it needs to be built up to, that when we-the-readers hit the bomb there's enough tension around it to pull us into the next chapter.
Q: Can you break up the climax?
Acknowledgements: Many thanks to @micascotti, @Kaori_Ino, @Evie_Redding, and @Azuarc for contributing questions to this post.