The Licht cycle by Stockhausen which consist of 7 operas that goes for 29 hours of music (although the MP3 I have is 34 hours). Each opera representing the day of the week.
I have to say I'm disappointed that he didn't have the length of the individual opera to go for exactly 24 hours with each opera last a single day and the cycle goes for a week.
I actually have the MP3 sitting in my computer (It was in TalkClassical forum Top 200 Operas and I managed to obtained every composition in all of their list). I don't think I'll ever listen to it though...