The existing "sitemap.xml" file is updated automatically every time you create/delete/edit a post/page or add a new post type/taxonomy to your site on the settings page.
If your site has less than 10,000 posts/pages then "sitemap.xml" file is updated immediately after you create/delete/edit any item. If it has more than 10,000 posts/pages then it will be updated in 10 minutes provided that your site has a traffic ranking. In other words, if your site has more than 10,000 posts/pages, the update of your sitemap.xml file will start in 10 minutes after the next user visits your site.
This article related to Sitemap by BestWebSoft plugin
Additional resources: Sitemap - User Guide
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