I have a nasty habit of watching DVDs in blocks - 2-4 episodes of something at a single sitting. Depending on the show, this may have... side effects...
Too much West Wing (4 episodes or more) definitely warps my speech patterns and sense of humour for a little while. But it's remarkably easy to watch...
More than two episodes of Deadwood and it starts to get to me. Somehow. Or maybe that's just the accompanying whisky...
Two episodes of Mad Men seems to be about as much as I can watch at once.
And I've never been able to watch more than a single episode of Six Feet Under at a time. It's just too bleak - mostly due to the way it's filmed, I think.
I've just started watching season one of Dexter. I think I could happily watch quite a lot of this in a single night. I'm not entirely sure what that would do to my brain, though...