Somewhere between three and four hours remain until someone Harry leaves, and it may be further until someone notices that he is gone.
It's June 13, 1992. The moon had already risen by the time they got to the graveyard ("The moon above was over three-quarters full, already seeming bright with night not fully fallen." Ch. 111). Hogwarts is located in Scotland, and the moon tables for that date show that in Glasgow, the moon rose at 8:48 pm, local time. It's not quite dark yet. The resurrection of Hermione and Voldemort recovering his body take some time.
By the time the Death Eaters arrive, it's night ("The gibbous moon riding higher in the cloudless sky, the stars and wash of the Milky Way visible in all their majesty within the darkness" Ch. 113). At that date the sun set at 10:03 pm in Scotland.
Now, at 11:04 Harry checks his watch and shortly thereafter receives the (fake) message. Harry goes back in time 5 hours and is confirmed to be in the past at 6:45 (11:45 future time). I don't know how long it took to possibly talk to Cedric and walk all the way back to the castle and make it to the third-floor corridor in time. If he gave himself some 10-15 minutes travel time, he's got at most an hour and a half before his departure from the Quiddich field.
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 113.
There is a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author’s Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
IMPORTANT -- From the end of chapter 113: