- "Father has fallen into the Odinsleep."
The Odinsleep is a state of deep sleep where Odin recharges the Odinforce (a magical force that gives Odin his power) and is left as vulnerable during the process. Odin is also completely aware of his surroundings and what is occurring throughout the universe, from Asgard to Midgard in his Odinsleep.
Not long after, Frigga visited Odin's bedside while he was still in the Odinsleep with her adoptive son still present. When Thor found Mjølnir on Earth but failed to claim it, Thor was killed on Earth by the Destroyer; Odin sensed this and a tear fell from his eye.
Later, Laufey tried to kill Odin as a part of Loki's plot, but was double crossed and was killed as a result. During Thor's battle with Loki on the Bifrost Bridge, Odin woke from the Odinsleep and tried to save them from both falling off into the abyss but lost Loki in the fall, presumably killing him.
While Odin is in Odinsleep, Loki asks his stepmother Frigga how long will Odin's sleep last. Frigga says that she doesn't know and this time they were unpreapared.
In short, the duration of Odinsleep is not constant and changes from time to time.