At least not much. They scan.
Particularly if they’re well-educated, high-literacy visitors to a Web site.
I was reminded of this bit of well-documented research in a recent post by Jakob Nielsen. In this post, the king of user interface went on to report that the average reader reads just 120 words per page. (FYI, that was 53 words…if you’ve even gotten this far.)
So, the big take-away for Web content is to Keep It Short.
PS: Here’s a related post.