The frequency that Google updates your site is based upon how important the Googlebot thinks your site is. Plenty of people are in the every 6 month boat - I've also read articles from people that literally can't get it to leave, and track it on their AWStats there just about all day everyday.
SEO is definately a science and a completely legitimate full-time profession for many. Back on the topic of submission, some experts say to submit once, manually, to get included, others say submitting at all is a bad idea.
Googlebot is contantly crawling the web, as are the other spiders, and if you can get your site included without submission then you really do never need to submit. Google's submission page says there's no penalty for submitting your site, or even doing it often, but there's still a lot of skepticism (that doesn't mean there's not a benefit for not doing it, right?). I recently read a very interesting article on SitePoint on an optimization strategy whereby the SEO'er submitted acquired an inbound link from a related pagerank 6 site. According to this article (of which I'm too lazy to legitimately quote), pages that are ranked pr6+ are typically viewed as the 'authority' on their topic, and an immediate link from such a site is the absolute best way to get your site initially listed.
Corey Northcutt: Freelance Internet Promotion Consultant