I try to aim for the 300-400 page range as I have found with longer books they tend to waffle in the middle to increase the page count. I have remarked a few times after finishing a 500-600 page book that the story could have been shortened by 100 pages or so with no loss to the storyline.
I reckon authors think that longer is better rather than providing a shorter,sharper story.
The Dome comes to mind(approx 800 pages) and my workmate who took ages to read it said he almost quit it twice
