no need to update site map frequently , only when u add more web page to your website then u need to update .
If you have new pages or updated the title or do redirection on your pages, you need to resubmit your xml sitemap to search engines. In case you just updated a content without changing the title or URL, search engines will crawl it even if you don't resubmit it. If you are referring to article sites, search engines will automatically crawl it since they often do this on websites with fresh contents.
Basically sitemap is an XML file that lists all available URLs for a website; therefore, when you add more page you need to upadate that.
Site map updating will be required only when you add some new Web Pages in your website. No need to extra effort.