Saw Bela Tarr's Satantango at a film festival a few years ago, same bum numbing 450mins as Melancholia and also split into three. It's an impressive feat of cinema, but have to admit i did get bored and fidgety in parts.

The NOES doc has been mentioned, there's a F13 one thats quite long - Crystal Lake Memories, & 'Video Nasties: the Definitive Guide' is almost 7 and a half hours.