The extended amount of time spent viewing consecutive episodes of a single TV show over the span of several hours, using the on demand feature of a cable service provider or subscription based service such as Netflix. Demandathons typically last for several hours or all day and can only be periodically interrupted for extremely necessary functions.
Clarkay: I just heard Erker exclaim his excitement regarding the newest season of "How I met your mother" becoming available on demand. Looks like he'll be in for another demandathon today.
BK: I'm not surprised to hear that, I'd say between that and whacking off a few times he's got a pretty busy day ahead of him.