The answer is different for every writer and every book. Or it should be.
Right now the digital book has excited readers. They enjoy reading and want more books. Writers want to provide more books because serving their audience is rewarding and it's also nice to be paid for the work you do.
A question we might be asking is whether a book equivalent to a Snickers bar or is it a meal?
If it's a Snickers, then a book just needs to offer quick, sweet satisfaction. If a book is a gourmet meal then time must be taken to prepare the offering. Thought and planning goes into a meal as well as various ingredients such as carefully chosen spices, herbs, and proteins to create layers of flavor.
It takes time to create something delicious that will be remembered as a unique experience.
How much time is required to fit all the elements together so they blend harmoniously and leave a memory of being touched by a significant experience? I don't know. It takes the time it takes.