Dungeons must be completed to get full credit. Logging out, leaving through the Dungeon entrance, going to another dungeon, playing a Mini-game, going to a PvP or Derby match, or going to your home will reset the Dungeon. Leaving any other way gives you 30 minutes to return before the Dungeon resets.
Dungeons have entry Sigils. If the sigils are lit up, you can enter the dungeon, just walk in to the blue circles, press ‘X’, and wait for the countdown to finish. You can also enter Dungeons with up to four players. To do so, you all have to interact with the circles before the countdown reaches zero.