People often assume that changes they make to their website will instantly be picked up by Google. This isn’t the case. It can take anything up to a few months before Google indexes new content on your site, although this would be an extreme case.
Search engines use “spiders” – automated systems that hop from site to site, following the links they find to discover new content and add that into their databases. Existing sites that are already in their databases will get revisited periodically to check for changes. The frequency that they revisit depends on how often the content on your site changes, how easy it is for them to index that new content, how well ranked your site is and how frequently you have asked them to reindex*.
3 days after switching on my new web site redesign, all the new pages have been indexed. The old pages haven’t however been removed from the index, so it was important to have redirects in place for those old pages to make sure visitors didn’t get “missing” 404 pages.
The lesson in this is that Google aggressively finds content. Anyone that tells you resubmitting your web site frequently to search engines is a good idea is usually just after your money, clueless or both. However, there’s little harm to be done by doing occasionally, so if after a couple of weeks your new content isn’t being found, give it a try. Just don’t pay someone to do it regularly as part of an Search Engine Optimisation plan…
* You can set this via meta tags