This is a question we often get when planning on launching a new Website for an organization. Much of the time we are taking down an older site and replacing it with a new one at the same domain. The older site more than likely had been indexed by Google and the new one probably has a completely different site structure beyond the home page. Though we can't answer this question with a specific amount of time, I can give you a short case study.
When we launched our new site one month ago (www.projectbigfish.com), we had roughly 140 pages or URLS in our site. After approximately 2 weeks, approximately 60 of those pages had been indexed by Google and one month after launch, our site had 174 pages and 118 pages had been indexed. During that same time, the older site URLs were being removed from the Google Index. At the two week mark, there were approximately 50 URLs from from the old site that were still indexed by Google and at the one month mark there were only 13.
We track this by utilizing Google Webmaster Tools, which on a daily basis updates these and other stats so that we can see how our site looks to the Google index. The service free and has been around for a while. We start by providing it a sitemap which is an XML file, that basically lists all the pages within the site. This serves two purposes, the first being that it tells Google what pages you would like indexed rather than waiting for it to find them on its own and the second being that it allows us to track the indexing process.
Webmaster tools also provides other features, including the ability to request that certain URLs be removed from the index. I used this feature to speed up the process of removing the older URLs that were no longer in existence.
