How often should I regenerate it?

Update it after publishing or deleting pages, changing URLs or moving a website. Refresh image sitemaps when image locations change. This tool does not schedule updates; a maintained sitemap from your website platform may be a better choice for frequently changing content.

Regenerate after URL renames, content removal or an image-host change, then publish the new file. Merely downloading an updated copy does not change the version currently on your server.