There doesn't seem to be a time limit for indexing a book, but life can get in the way!
There was a thread about this a few years back and the answer was to email EYB (probably Deborah) so that they can contact whoever is indexing it and see if they wish to continue. If not (and you are prepared to take it on) the book can be unassigned then you can offer to index it. You do get access to everything that has already been indexed for a book, so you're not starting from scratch, although when I did this the previous indexer had only done about 5 or 6 recipes. I think sometimes people decide they want to index a book but then get overwhelmed by the task.