Klove,
It depends on "how important" Google thinks your page is..
and the content on your page,
and how often it changes,
and how often the page is updated,
and the no. of incoming links,
and the quality and relevance of those incoming links..
and..
and..
and...
If it is a single page, with a few incoming links, and the page is not updated daily etc... then as per what GG said.. about every month.
DD