A dungeon crawl should probably last until the players or the DM are tired of it. I've had them last 5 or so sessions, which would be 5 weeks real time.
I think when we played the Aerie of the Crow god that lasted longer than 5 weeks, but I could be wrong. That however really seemed to have a good story behind it and part of it didn't seem to be, "Oh the bad guys decided to make this there lair and dug it out for several years."
In the Ptolus campaign I'm in our dungeon delves, which have all been in the sewers, except one, have lasted 1 or 2 sessions and I was disappointed at least once because as soon as we got above ground the game ground to a halt while we decided what to do next.
I think when we played the Aerie of the Crow god that lasted longer than 5 weeks, but I could be wrong. That however really seemed to have a good story behind it and part of it didn't seem to be, "Oh the bad guys decided to make this there lair and dug it out for several years."
In the Ptolus campaign I'm in our dungeon delves, which have all been in the sewers, except one, have lasted 1 or 2 sessions and I was disappointed at least once because as soon as we got above ground the game ground to a halt while we decided what to do next.