Google does an initial crawl of the home page and some links from it. At usually only goes one level deep unless there's a link with keywords common to the home page. Then it
might go two levels deep. Seems to prefer high-on-the-page links during the initial crawl - good idea to put your site map link high on the page.
3-9 months after the initial crawl, it comes back for deep crawls - usually spread over a period of time to minimize the bandwidth hit on your site. It will return for the deep crawls faster if a large number of pages which are ranked highly for your search terms link to you.