I don't know how long they actually were, but both Dances with Wolves and Saving Private Ryan seemed about 17 hours long.

It’s easier to sit through long movies these days, but at the time it came out Schindler’s List was a real trek. My brother and I brought full McDonalds meals to the theater with us. Excellent movie, though.

Re: what is the longest movie you have ever had to sit through?
 

2012 movie. It was the longest movie I watched not counting documentaries. But want to add that this film deserves the time you spend on it. Very strong movie.

I regard the whole LOTR trilogy as a single 11 hour long movie (with the extended versions), so that is the longest.