I am writing my first chapter books for an older audience around 18+ and I need a little help with knowing when one characters point of view should end and the next begin. Can anyone help me?
If I understand the question correctly, we are being asked when POV should switch, and this question also assumes that POV will switch at chapter breaks.
Typically, POV should switch when a scene ends, and the subsequent scene would feature a problem that will have greater stakes for a different POV character. POV is usually best served to the character who has the most to lose in a scene, and so when the greatest burden of the stakes shifts from one POV character to another, then so should the POV.
As for chapter breaks, well, that's largely a matter of opinion. Something to keep in mind, however, is that a chapter break can serve an author well when there is a hook that will draw the reader on to the next chapter. Breaking a chapter after a peak of activity or tension may draw the reader on in an effort to find out what happened next.
The number of pages is as many as are needed. As Bryan shows, a chapter can be as short as a word, or a sentence. it can be very long.
In general, most chapters are a continuous narrative (a combination of exposition and dialogue) of sequential action. It could be exposition that covers the highlights of a thousand years, or it could cover a single conversation with a stranger in a parking lot.
The chapter ends when you need to "shift gears", a break to something else, when you can't write any more useful things for the reader to have in THIS narrative, and you wish to start another.
For example, the chapter ends when they board the train, there is zero plot development or character development you wish to do on the train, nothing you want to reveal in dialogue, no scenery worth describing that helps with the plot or foreshadowing or anything else, so the chapter ends. The trip is uneventful in the literal sense. The next chapter begins, "They finally found the Knightsmark hotel in Tragerton, where ...".
(IMO) Your writing should always be concise and not wander into sideshows much; everything I write is to advance something about the plot or characters. So I do not write about spans of time in which nothing advances, like when my main character is asleep, or traveling alone, etc.
Such spans of time are generally good times to break the chapter, and/or POV. Readers may not exactly be expecting a chapter break to cover a large span of time, but they are at least accustomed to that happening, and the next chapter beginning "[some_time_span] later, ...", whether that is minutes, days, weeks, months, years, a lifetime, whatever. One chapter ends with a baby being born. The next begins "John grew into athletic young man, unfortunately already balding at twenty-five."
The time span is certainly not necessary, just a POV change is fine. "While Allen was doing that, Bill was still puzzling on how to get into the safe."
You change when the setting changes, the POV changes, or uneventful time needs to elapse.
Do not cut or add words to try and reach some "good length".
I found an interesting Blog post on this. https://blog.reedsy.com/how-long-should-a-chapter-be/
Based on their review of different books, the average word count is between 3000 and 4000 words. That being said, it's up to you. As long as you have covered everything it makes sense to cover in that chapter, then you are done.
For the fun of it, I looked to see what the shortest chapters in books are. I found this lovely gem out there. https://www.shortlist.com/entertainment/the-20-shortest-chapters-in-literature/99631
Forgive me for nitpicking, but I can understand anything in this.
18+ could mean 75 years old, and you don't write the same way for that age group as you would a 18-35 year old target audience.
That aside, there is no hard and fast rule. Sure, there are guidelines. One such guideline states that each scene should be roughly 3500 words. But adhering too strictly to that gives you problems.
Let's say you're in a pivotal scene, and you're now at 4500 words? Do you break off the scene? Start off in the next chapter? Why is "1 scene = 1 chapter" a rule?
Truth is, it isn't. And how long or how short each scene is depends on what you want to show.
And example from Arch Angel's Shadow (by Nalini Singh). One chapter was ten pages long (roughly 5000 words). Another was a single page (rought 500 words). Why? The shorter chapter was describing the antagonist, vaguely showing they were waiting, waiting for the right moment. That the victim was already growing weak, her skin thin as paper and almost see-through.
Those 500 words hit you. Hard. You get a sense of urgency, a sense that it might already be too late. That it probably is too late. Another victim is going to die, and you don't want that for her.
In those 500 words, Singh achieved what she wanted to. Why drag it out? End the chapter, end the scene, move on. And leave your reader with that ball of worries in the pit of their stomach. If that is your goal, never offer a word more than what you need to achieve it.
15 pages per chapter, is my answer. You can do your own thing though. Also, next time you write a question, you should be slightly more specific. Like other comments pointed out, you should specify what age. You said 18+, but that could be 19 or 92. You write differently for those two ages. I'm very late, but this might help other people who come to this question.