There's a potion too, right? (Whatever's in the cauldron before Wormtail's hand is added, although that might have only been in the movie.) If that has to be made before the grave is robbed and goes bad after a few hours, then you have a time limit.
I had forgotten about the potion.
Still, I don't think he would have bothered breaking Bellatrix out if he didn't expect to use her in the ritual. I also can't think of a reason to have her hide his wand next to his father's grave unless that connection was enough to keep the bone potent.
Update: Discussion has moved on to a new thread.
After 61 chapters of Harry Potter and the Methods of Rationality and 5 discussion threads with over 500 comments each, HPMOR discussion has graduated from the main page and moved into the Less Wrong discussion section (which seems like a more appropriate location). You can post all of your insights, speculation, and, well, discussion about Eliezer Yudkowsky's Harry Potter fanfic here.
Previous threads are available under the harry_potter tag on the main page (or: one, two, three, four, five); this and future threads will be found under the discussion section tag (since there is a separate tag system for the discussion section). See also the author page for (almost) all things HPMOR, and AdeleneDawner's Author's Notes archive for one thing that the author page is missing.
As a reminder, it's useful to indicate at the start of your comment which chapter you are commenting on. Time passes but your comment stays the same.
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: