I am writing a second chapter for a book with another author.  It is quite a lot of work.  The first chapter I wrote took about two months with all of the research.  Of course if you have the time to devote to just writing it would go faster, at least as far as the calendar is concerned.  It is the little things like work and family that tend to slow things down. 

In all it is a fun experience and it actually makes you better at the subject you are writing about.