Matt Cutts takes a fresh cut at trying to explain Google algorithm updates and data refreshes. In a nutshell:
- Algorithm updates affect the methodology that Google ranks sites. These are very infrequent.
- Data refreshes involve changes in the data set which the algorithm processes. One example of a data refresh is an index update. The index is updated daily.
What this means is that search results are constantly changing and evolving. The results you see for given search terms are changing on a dynamic basis. This is a fundamental part of the Google process now, that allows them to be much more responsive to changes in the web, in terms of keeping their index as current as possible.
The good news is that as long as your site is not penalized in some fashion you should be able to see the results of your marketing campaigns in a relatively short period of time. Links from higher page rank pages on higher page rank sites should be found very quickly by Google during their normal crawls. Lower page rank links will take longer for Google to find.
But if you get into a good rythym of getting new quality links to your site, you should be able to see you site move up in the rankings on a steady basis.