Sitemap updates frequency
You should update your News Sitemap as often as you update your news site with new news articles. Google News crawls your News Sitemap and your regular site with the same frequency.
Please keep in mind that you should only list articles in your News Sitemap which have been published in the last two days. You can remove articles older than two days from the News sitemap, but they will remain in the News index for the regular 30 day period.