I have spent four or five hours watching video tapes or DVDs. There are some nights when I get home from work when I'll turn the TV on at 6 PM to watch Cash Cab, switch over to watch Jeopardy! and Wheel of Fortune and then watch two or three hours of prime time programming. So I'd guess four or five hours is my limit.
Mostly I watch shows because I'm interested in the content of the show. Although sometimes I turn on the TV to have some background noise. Occasionally, I'll sit in the room where somebody else is watching TV while I do something else (like websurf or read.)