How often goggle refresh pages?

[seo.gops]

Hi friends,

I'm very new on this and I have a lot of doubts. I was reading other answers and concluded tah Keyword tag is not very usefull. The best way to increase possition in google and others search engines is put the words in the body of the page, is this correct?
The other doubt I have is, once I upload the modified page with the new keywors or the new body design do I need to re-register the site or goolgle updates this information? How long it takes?

[Electrum]

You can make your site visited by Google by using an XML sitemap and in that, you will be specified the frequency of updates. But usually, it will take a week to crawl or you can crawl your changes manually by using Google Webmaster.