For an average corporate website, google usually visits the homepage of the site in every 10 to 15 days and for the interior pages it will be less frequent.
Every time Gooogle visits a site, it reviews whether the site has been updated or not.
If the website is updated regularly, chances are more that Google will visit the site more frequently.
For frequently updated websites like directories, or classified advertising sites etc.
the number of visits by the search engine may increase to many hundreds or even thousand of times each day.