I can tell you my experience.
The fastest I've published a book was Twist, FIVE months after the release of Sterling. I worked hard, and actually little had to be done with this book. But Impulse took EIGHT months. The editing slid back and it was an intensive book to go through. Gravity is taking about SEVEN months. (Seems to be my average)
You may know of some authors who can knock the books out each month. There are reasons behind that. Some are first draft writers. Some don't have editors or proofreaders. And most of those writers also write full time, so they can devote more of their days to writing.
For me, I knock out my first draft pretty quickly, but I really love the revision process that comes with drafting. It's particularly important in my series, because I spent a lot of time on details that connect the book with past and future books, and I have to be careful about how I weave them in.
- Writing/drafting time: 1-5 months
- Beta read: 1 month
- Editor & proofreader: 2 weeks - 1 month
Unexpected things can also occur in sliding the release date back that can't be helped. The thing is, readers want those books quickly, and I totally get it. But some of those books that are released back to back readers also complain about. Too many errors, a rushed ending, the story felt rushed. This doesn't just apply to Indie, but also Tradpubbed authors as well. Once they start rushing those releases (in comparison to their usual release cycle times), I often find that the stories are less rewarding.
But like I said, every author has a different process, and so many variables factor in. I do hope to write full time one day, and if that happens, I am positive that I'd be able to knock out more books per year!
But I'm actually happy with 5-8 months. I think that's pretty great compared to over a year. Last year, I published three books because I squeezed a novella in there. Do I wish I could get them to you faster? Of course. But when I compare first draft to fourth, there's a remarkable difference, so the drafting time is necessary. I'm not referring to typos, but scene structure and flow, character development, and plot outline.
So that, in a nutshell, is why publishing takes so dang long. I don't think a reader will lose interest in an author they love simply because of the time delay in releases. If you enjoy someone's writing, that's pretty much it. Signed, sealed, delivered. I have my own authors I read and follow, and even if it's five years between book releases, you bet I'm the first in line to buy that next book. :-)