Originally Posted by http://www.google.com/support/webmasters/bin/answer.py?answer=34439&topic=8843
How often does Google crawl the web?
Google's spiders regularly crawl the web to rebuild our index. Crawls are based on many factors such as PageRank, links to a page, and crawling constraints such as the number of parameters in a URL. Any number of factors can affect the crawl frequency of individual sites.
Our crawl process is algorithmic; computer programs determine which sites to crawl, how often, and how many pages to fetch from each site. We don't accept payment to crawl a site more frequently. For tips on maintaining a crawler-friendly website, please visit our webmaster guidelines.
Some years ago, Google would update their main rankings engine maybe once every month but now are now very active and do the regular updates in the websites rankings nearly everyday.
Google send out their bots to crawl sites on a regular basis. A good proof of this is seeing how new contents from blogs and even forum posts gets indexed almost instantly.
Google’s spiders crawl the web on a regular basis to rebuild their index, spiders tend to visit a site more often if the site is updated.
Quality backlinks also help major search engines’ crawlers find your site and can give your site greater visibility in their search result.
Every how many months google spider crawl the web....???
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