I am somewhat embarassed to admit that the last show I watched for 7+ hours was America's Next Top Model. I left it on TV as background noise. In my own defense, I only watch it on MTV reruns.
I don't like TV-aired marathons, because I feel like I'm stuck in a room for hours on end, and if I leave I'll miss something. It's one of the reasons I love buying favorite shows on DVD. I can direct my marathons and choose how many eps I watch in a row.
However, for shows that aren't on DVD yet, I'd like to see end-to-end marathons of: Special Unit 2, Medical Investigation, Swamp Thing, The Others (show, not movie), The Fugitive (Tim Daly's remake), The Nine (damn you, ABC!), Brimstone, and Now & Again.