As you might guess, the answer is "it depends." I tend to read fiction cover to cover, and if it's short enough (the average Dean Koontz novel, for example) I often read it one sitting. A Stephen King novel like
Under The Dome might take 2-3 sittings.
Non-fiction is a different story. I tend to read business books cover to cover (relative to a span of time, like say a week or two), but I interleave reading multiple books so that, at any given time, I have 3-4 books I'm in the middle of reading. Which one I pick up to read a few pages of on any given evening when I have time to read just comes down to a whim.
Similar case with other non-fiction stuff... I have bookmarks stuffed in probably a couple of dozen books that are in some intermediate stage of being read. But sometimes one just catches my fancy and I knock it out in a few days (or whatever time-frame, depending on the length and difficulty.)
The big exception are books that are basically just reference. A tutorial'ish book like Grails In Action I might go through almost cover to cover, but at some point when I've learned enough to start "doing stuff" I tend to start skipping around. Books like that I often never technically "finish." Other reference books I pick up, skip to the section I care about, read that part, then put it aside and never touch it again. For example, I'm skipping around in Mahout In Action now, focusing on the section on clustering.
Sometimes I read just the part I need now then put the book in the "read this entire book later" pile. Then it just goes in the queue with all the other partially read books.
So, following this system the time span I spend on any given book might range from 6-7 hours (like the last Dean Koontz novel I read) to 20+ years ( I think I started Douglas Hofstadter's Metamagical Themas at least that long ago, and it's still in the "come back and finish this later" pile.)