For some reasons, I've always presumed that Google crawls websites weekly until today. Someone said its bi-annually. Is this true? If not, how often?
Once your website has got indexed, then your website gets crawled by Google... This is just my opinion...
There really isn't a set schedule. As far as I know, websites get crawled all the time and the frequency of a specific website can vary. You can also ping your website after making content changes or additions.
Well, we all know Pagerank. If your PR is high your website gets crawled very often. If I am not wrong a PR5 website is crawled at least one time per day.
It can crawl it daily, weekly or even longer. The more you write the more it will search. When I started out with one site a long time ago, it took Google weeks to include it in their search. It's now there in a few hours. Even quicker for more respected sites.
Google Crawl is dependent on your content and updates. Research shows blogs/news site gets crawled more than static HTML site with weekly update. More content they more google with show your site love.
you may login into google account and go to webmaster tools. add your site and setup crawling rates and use other helpful options to make your site noticeable for google.
If you keep pinging backlink urls that go your site, or posting material to newsy sites that link to it, Google should be crawling the target site very often.
If you update your content regularly, it will crawl every day. If you have a CMS, such as WordPress, it can crawl as fast as a couple minutes, because WordPress sends a ping to pingomatic, which in turn, notifies Google that you have new content, then it will crawl it soon. Use Google Webmaster Tools as others said, and that will help too.
As others have said, there is no fixed time, but it is significantly more times than biannually. Pinging definitely speeds up the process.