Nice catch JamesPoppell! I thought Y2G was talking about writing one. I guess it does depend on if you are writing or reading. Reading, it depends on my interest level in the book.
On writing, I would say it depends on your inspiration and motivation. I wrote a 55K word novel in a month, but I have another that I've been working on for about a year now. The first was very inspired and just flowed. The one I'm working on now...obviously I'm struggling with.