once you have submitted a sitemap to google you just need to keep the sitemap up to date google will check the file on a regular basis.
Well i will advice you to submit your sitemap every time whenever you update your website to let google know about the update taking place on the website. But there should time gap of 2 weeks between every submit.
Why you should wait two weeks to between sitemaps if your website has been updated more frequently than that?
I will suggest you spend the time on SEO than updating the sitemap. Getting backlinks is most important part of SEO.
i have a cron job update my sitemap every night at 3am, then another cron job pinging google to come checkout the new sitemap every 2 weeks
I resubmit my sitemaps immediately after site update. No need to resubmit every month, resubmission is needed only if you make some changes.
I use Xsite Pro to build my website. It always updates the google sitemap. But according to google, sitemap does not help in pageranking
Honestly, it depends on the site for when I ping google with the sitemap telling it there's an updated version. My forum pings it daily because the content is constantly changing, my web directory is quite new and I didn't start pinging google with the sitemap until I already had about 150 links listed, but now I do it about once a week. My blog automatically pings whenever I make a new post.