He needs 6 hours of uninterrupted time with Hermione's body. His present self guards the door while his future self does whatever he plans on doing to prepare her body for long term preservation.
See this quote from chapter 91 set right after Harry exits the room where her body is stored:
When the door opened again, Harry seemed to have changed, as though that minute and a half had passed over the course of lifetimes.
That "lifetime" is more specifically 6 hours.
"Lifetimes" could also be literal (though this is a bit dubious considering that it's from McGonagall's POV) - perhaps Harry managed to reanimate Hermione for multiple brief periods? Or, perhaps Harry experimented with animating, killing, or reanimating other, smaller creatures?
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 chapters 91 & 92 . The previous thread has passed 500 comments.
There is now 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.)
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17,18,19,20.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: