Not sure exactly, but I started on Saturday and read half of it, then read a little every day to finish it. It went faster than it should have for two reasons. First, the writing is very good which makes it easy to read and comprehend fast. Second, half of the story is in the past and half in the future. The part in the future makes use of many existing star trek stories, and when I got to those parts, I just skimmed those sections. Anyone who knows the TOS episodes and movies really only needs to read 3/4 of the book and can skim the other 1/4.
My best guess is about 8 or 9 hours.