I can’t imagine writing a fictional book with all the twists and turns, foreshadowing, character development, etc. That is a special talent!
For my book, the stories and lessons came fairly quickly. But I have chosen to get each devotion, verse of the week, and music on just two pages.
So the toughest part was getting the stories edited just right to be short enough but still understandable. It was actually fun coming up with the actions and searching for the music 
But then . . . constant editing: word choices, phrasing, grammar, etc. I started writing in the summer of 2020. The book is supposed to be published in January 2022. I’m guessing this is about average. I’m sure others do it more quickly, and others may take years.
It’s been an amazing process & I’m learning more with each and every day 