A Google Webmaster Help thread asks how often does Google crawl your XML sitemap file. Google’s JohnMu replied saying it depends on how often the file is updated.

John said:

In short, John said “Google’s Sitemaps crawler usually reacts to the update frequency of your Sitemap files.”

This is pretty much how Google decides to crawl your pages. If you update your pages frequently and you have a normal PageRank, Google will likely crawl your web site more often.

I should add that more web site crawling down not necessarily mean higher ranking in Google. It may be fresher content from your site being included in Google, but doesn’t mean you will rank higher.

Forum discussion at Google Webmaster Help.