Tough one. I read about a film released in Bombay called Mohabbatein by Yash Chobra which was about three hours 40 mins long. But so many movies are epic these days. ANyone else know for sure?
Heimat, with Marita Breuer and Willi Burger, produced by Edgar Reitz runs for 924 minutes (15 hrs 24 mins) and is about life in a German Village from 1919 to 1982.
A brilliant version , in Russian, of War & Peace which I saw in one sitting at my local Odeon about 30 years ago. It lasted between 6 -8 hours, I can't remember exactly.
"Until the End of the World" (1991) runs for 279 minutes. The full version is available on DVD, but the cinema release was shortened to 158 minutes against the wishes of the director.
The original version of "Lost Horizon" (1937) ran for 6 hours, but has since been lost.