Totally depends. Gödel, Escher, Bach took me at least a good six months; Zen and the art of Motorcycle Maintenance took a few weeks.
I have started Gödel, Escher, Bach three times now. I've yet to make it all the way though. I hate books that are supposedly fantastic, must-reads but can't hold my attention for more than a few days. It feels like the horribly naggy ex-girlfriend of failure.
If it's any consolation, it took me two years to finally finish that book. You can get a large part out of the book by just reading the dialogues if you are good at interpreting metaphors. Coincidentally, I was just in a presentation yesterday where the presenter went on a tangent to talk about that book!
There are a great many "great books" that are abject bullshit. Robert Persig, for example, knew jack shit about zen and knew jack shit about motorcycle maintenance (favorite part of the book: where he runs out of gas then realizes that his motorcycle, like every bike ever made, has a reserve tank in lieu of a gas gage). I was incapable of reading the Cliff's Notes for anything Jane Austen.