The dungeon I think is the longest is... the dungeon you have the most trouble in and get lost in and have trouble fighting the enemies. But if you don't do any of those things I would probably say the labyrinth. It took me 2 and a half hours to beat it. :?
Actually you can't say for sure what's the longest but by yourself, they are all hard. I would say by yourself or with a group, The TEST REALM WAREHOUSE - 12 floors of pure Mularky.
But if you take a Storm and and Ice or Fire Guy with you...It won't take long at all. Kensington Park is the Hardest but can be tamed like a Kitty with a group of people. Key thing people.. Always keep your MANA full so MARKING your spot is possible. I got left in Kensington Park with 2 or 3 battles left, but as a Dungeon no more can enter ..too far into it but who you kill stays dead for a bit..So I kept going back to get full health and mana and porting back in..It worked and I finally got past it..
Kensington Park, I've tried, and failed because I got extremely bored.
And if I had to choose the shortest...it would be the Tree of Life.
I think the Sunken City is the longest dungeon. When I tried to beat it the first time, I got beaten, along with a lot of other times until I got help. I think the shortest dungeon is the past Dragonspyre. That only took like 10 minutes.