I ran my 12 yo son (basically a newbie to TTRPGs) through most of BlackFang's Dungeon. We have not completed it due to lack of time to play- however he has managed to make it roughly 2/3 the way through in about 3 hours-albeit with some great dice rolls on his part that made a couple combats pretty short, as well as some good roleplay with the Goblins which kept him out of that fight.
I'd say the Dark Mistress' guess of 4-6 hours is spot-on.