David Degrelle, of e-SEMA (European Search Engine Marketing Alliance) tells me that Google is pretty quick these days, and his blog post – Google calculates SERP’s in 14 minutes and goes real time! points to a quick turnover time.
I’d be a little surprised if Google refreshed it’s whole index in that quick a period of time – there are so many websites to crawl, including new sites, and updated pages, that it seems unlikely. Yet, blogs that ping Google when there are updates may find the content of their pages getting indexed pretty quickly. And I’ve seen times indicated for news results at Google News that are minutes old, rather than hours.