The longest "mostly downhill" ride I've ever been on has been leaving Crater Lake (Oregon) and heading towards the ocean. It was "fast downhill" for the first five or six miles, and then it's "more or less" downhill for about 60 more miles. We'd be coasting along at ~25mph and then we'd start pedalling and get up to low-30mph range for a bit and start coasting again, etc. We covered the whole thing in about two hours.

And, for the purists out there, yes, we had ridden to Crater Lake in the first place. Different route but same basic thing -- 30, 40 some miles at 2, 3% grade.

The other one that comes to mind is Beartooth Pass on the Montana / Wyoming border near Yellowstone. The Montana side is 33 miles long. I forget how long it took to get down... probably the better part of an hour.