Easily Lord of the Rings. If you watched the Special Extended Limited Edition of all three movies without stopping, you're looking at staring at the tv for a little over 12 hours, for they're over 4 hours apiece.
^^ - El Diablo122
Sorry to disapoint a Hobbit fan but while the Rings series was epic it ONLY ran three movies and 12 hours as you point out....
even the old "cliffhangers" beat that at an episode a week of about 15 minutes comes to 13 hours for just one year (they continued from rescue of last weeks cliffhanger and ended in a cliffhanger to be solver the next week,Pauline was the harried-est gal ever imho)
While I cannot say 100% positive, for as the above response shows, just one example will disprove a claim but the one to claim this title will need to beat the Friday the Thirteenth (Original) Series which is 12 movies long and at a bare minimum of 90 minutes each, will have you watching for 18 hours minimum (probably over 20 hours as I'm sure some were 2 hours long)