Have you read Dune
yet? That's a really good audiobook with an ensemble cast, runs a little over 20 hours.
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.
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.