Google isn't half as stupid as some people sure seem to think it is. Use their webmaster tools, submit it once and watch it - you'll find even months later it comes back to you every month without fail and without you telling it to. Just keep your sitemap updated and if you do make a major change I suppose it's okay to resubmit it, but minus that, you don't need to do anything.
For example, I have not updated the sitemap for my
Nina Mercedez site in months. I know, shame on me but i've been so busy with Content Bandit.
Well despite my no updates, Google on its own downloaded my sitemap and did its thing on December 1st. Prior to that it had downloaded it like on November 17th, and that's all without me ever touching a thing.
Now my
Sunny Leone site, Google downloaded it on December 1st and then again 17 hours ago. That's twice within the first 3 days of the month, with no updates at all on my end mind you. The previous month I got one visit total which was like on around November 5 or around there. I forgot the exact date now.
There is no set rule as to when it will come, just know that it will come and you don't have to keep pressing resubmit to get it to you.
Watching my stats I have found that Google does a quick run by or swipe and sees what is up with my site. If nothing has change it moves on but if I do have changes it seems to register those changes and them comes back later on for a closer, more in depth looks, aka a deep crawl.
Long story short, if you update your website, update your sitemap and Google and other engines will find it and do their thing with it.