I have experienced posting a question, then searching google immediately after for the core issue of the question and having that question pop up as a result of the search.
So Google may be aware of a question posted within 60 seconds of it being posted. I haven't done any testing to find out if this is always true, but when Jeff has discussed Google bot activity on the site, and how they update the sitemap (ie, it's dynamically generated, and google slurps it at least once a minute, if not more frequently) then it is likely that you may as well assume Google has what amounts to a direct feed for new questions.
However, answers do not receive the same level of attention.