As with any good Web Developer, the ability to time the changes Google will update your website and refresh the content for better SEO (Search Engine Optimization) is in your favor. Welcome to the world of "Google Dance". The Google Dance is simply that the predetermination of when the actual update will commence.
What really happens is Google sends out spiders to crawl the Internet, usually done to DNS (Domain Name Servers), upon spidering all of the available tables it begins to go through each individual site and updates the content on to Google.com. Thus if you watch your rank on the Google Toolbar, you can tell when your overall page rank has changed.
Understanding SEO and what is necessary to improve your overall ranking. Webmasters have been looking for ways to increase the odds of guessing when the next spidering would commence. There are various versions and servers that go out and crawl thousands of servers at a time, it takes time to relay and decipher this information back to the web server that Google.com pulls its information from.
Some vendors have created programs that actually go out to the data centers themselves to find out approximately when the last index was kicked off. Depending on the information returned from the trace the exact time and date are pinpointed. Is there really that much of an advantage to doing it this way? It depends, if you have a vital update that you want to optimize your site, you may want to know when the last time Google visited your site for content to keep information fresh and relevant.
There are many data centers that Google uses to spider across the world. Each center has a specific region it covers and all the information is gathered in aggregate and returned back to populate Google.com. With this many data centers the chances for continuous indexing is good, but not guaranteed.