It can take anywhere from 6 months to a year or more to ghostwrite a book. It all depends on the length of the book, the subject matter, the availability of the person or persons being interviewed, and the writing style of the ghostwriter. A shorter book can obviously be completed in a shorter time frame than a longer book. And a book about a simpler subject matter will be easier and quicker to write than a book about a more complex subject. If the ghostwriter has easy access to the person or persons who will be providing information for the book, that will obviously speed up the process.
And finally, some ghostwriters have a faster writing style than others. So, all things considered, it can take anywhere from a few months to a year or more to ghostwrite a book.
