Depending on how free time I have, I can spend about three or four hours reading. This is with interruptions and distracting things like the Internet.
I think the longest I've gone with straight reading is about three hours and that was mostly because I was left home alone with nothing to distract me.
Also, it depends if I have books to read that week, how good the book really is, and the level of difficulty I'm reading at.
Example: It takes me, eh, one or two hours to finish one of the Percy Jackson novels. Piece of cake. I can finish the Harry Potter series in less than a week, if I'm in a re-reading mood.
It took me about three days to finish off Jane Eyre. It was old English, so you can't really blame me too terribly on that one.
Right now, I'm reading a book filled with Sherlock Holmes shorts and one of the novels. I've had the book a week and I haven't even gotten to page 200. (This is extremely shocking to me. I normally read extremely fast.) The fact that it's made with two columns per page might contribute to this fact as well.
But, anyways, three or four hours, depending on the factors I previously mentioned. :D