How long should Deadmines (Lvl. 15-21) take?

I have read that dungeons should take 1-2+ hours, but when I do dungoens with a normal group of people, it only takes about 30mins or less, so does it sound like it should take that long, or am I doing it wrong? I do the deadmines with the two-headed orc, the goblin, the Foe Reaper 3000, Captain Cookie, Admiral Ripsnarl and I may have forgotten some. Thanks for answering...