Whenever google wants to
from:
http://www.searchenginejournal.com/index.php?p=2333
Google Update on the Horizon
Remember the old days of the Google Dance? Yes, those were the days. Like clockwork, at the first or end of the month, Google’s index would update with new listings and backlinks as site owners and the SEO community would do the Google Dance of Joy, or retreat back to their caves to tweak their sites for the next thirty days - hoping to achieve higher rankings again.
Nowadays Google more or less updates their index on an ongoing basis, adding and reranking sites on the fly, with mini-major updates every so often. Matt Cutts from Google says that once of those mini-major updates is on the horizon:
Just to give you a heads-up, I think a new set of backlinks (and possibly PageRank) will probably be visible relatively soon; I’m guessing within the next few days. I still expect some flux after that though, just to let you know.
http://www.mattcutts.com/blog/more-info-on-updates/