Stephane Gesbert, who wrote the original CDA, has free scenarios linked up at his webpage. I'd say that's a good 50+ hours of gaming right there. I've run a group through it. Great fun and high quality clue trails.
Other than that I have run a heavily modified "Here Be Dragons" which was written by Dean P. Goodwin. It's normally 2-3 hours but I made it about 20. As a result it's now a miniseries when I run it. I'm running it for the 7th time for a group online.
I have a couple of other scenarios on the back burner, including the Siege of Paris by the Norsemen and the quest to bring Brian Boru back from the dead. He was the Charlemange of Ireland, btw.
Of course the challenge is that there are so many settings that this all takes time and CoC isn't the only RPG that I play.