Have you read
Dune yet? That's a really good audiobook with an ensemble cast, runs a little over 20 hours.
Hyperion by Dan Simmons is also really good. Also an ensemble cast as I recall. Also a little over 20 hours.
If you want to go insanely long you could always do Ken Follet's
The Pillars of the Earth. Yeah, not SFF, but it's 40+ hours for 1 credit.
Brandon Sanderson's
The Way of Kings is a whopping 45 hours for 1 credit. That's the first book in a series he began two years ago. I think it's the best thing he's written so far.
And then there's Patrick Rothfuss. The first book of the Kingkiller Chronicles,
The Name of the Wind is 28 hours for 1 credit, and the second,
The Wise Man's Fear, is 43.
That's all I can find in my library that I like, that's 1 credit, and that's looong.
Oh, oh, one more. Neil Gaiman's
American Gods. That's the original version with George Guidall, which I love. They did an ensemble
10th anniversary version too, but I've only listened to the Guidall one. Both are in the 20 hour neighborhood.