This is purely hypothetical, just for fun. In this scenario, you don’t get to know who the GM is in advance, but they do have to run it as written. The players can leave whenever they like, but the GM has to stay until either all players leave or the players beat the dungeon.
The BBEG is located at the center of the dungeon. Each sealed room of the dungeon is guarded by a magic djinn which will only open the next room if certain fetch quest items are collected. All of the required items are located in the dungeon, but require the PCs to traverse trapped rooms to get them (you now how PCs just love dealing with traps).
There are 100 sealed rooms to get to the center of the dungeon. Players can take a long rest after each room.