that's a good question! The haunting events happen at 30 minute intervals, starting as soon as the sub is underwater (even if just barely under the surface, there's no need to go deep). I would say that the british captain npc would ask for a dive of a test dive of a few hours at least. In play tests the crew got immediately involved in the paranormal happenings, many of which pressure them either to surface right away or stay below to keep investigating, so the planned length of dive was disregarded. If it needs to the sub can stay underwater for 24 hours before the batteries start to die.
i'm so glad you like the game, thanks!