whenever you introduce new pages in your website. you can upload a new sitemap that contains fresh links.
You need to update them when you add new pages to your website, it's not greyhat, it's whitehat and Google want you to create/submit sitemaps.
There is nothing like limits for submitting sitemap, If your site is updated daily then you should submit once a day or whenever you post something new to your site. It helps google to find out all pages of your site, thats it.. Nothing more help to our seo.