They change as new sites are entered into the mix but a full update of the datacenters usually happens monthly

So let's say my site is ranked like #300 for a search term (I know, it's pretty bad :( ) and I do some onsite optimization. How long will it take for the site to change rankings (at least in Google)? Is it a matter of Google indexing the site or is it always a montly thing like you just mentioned? Thanks!

If I were to do changes to a site, how long generally would be a reasonable time to wait to see if there are changes or before doing more changes? Thanks!

Let's say you rank #300 on the term "Marketing Schemes" with one silly article. Okay, want to improve that ranking? - then first of all look at the article and the title of it and what could generally improve it? - I don't mean how keywords are placed in relation to others or smart hat tricks to make the SE think this is better than it really is, but if you found that article while searching for Marketing Schemes - what would you be really happy to see and read about?

A good title is also necessary to actually attract people and make them think "sounds cool" - with the phrase "Marketing Schemes and Fraud on the Internet" you have a good, basic and natural title - like in a newspaper. Describe the basic contents through the title - "What's this about"

I will also advice you to do some research on related subjects to "Marketing Schemes" - this could be common practises such as "Ponzi Schemes" and "Pyramid Schemes" and "Ways to Trick You into Buying" and so on.... make seperate articles on these subjects, and interlink all those related articles. That should improve the rankings fairly and expand your target market while you're at it.

And with those more or less specific keyphrases, it will also have a much longer longevity and quality in relation to other parts of your site and that will reflect on the rest of your contents and subjects on your site which will in general also rise in the serps because you did a good job.

I'm sorry, there is no quick way to consistent fame and fortune. But there is a way and it can be travelled. :)

Sincerely, and have fun.

Forgot to answer the question, how long time - it can be anything from a day to a couple of weeks. Depends on the link structures related to it - internal or external or both....

Sincerely, and have fun