how often google update cached pages?
 

who know? or never?

Well, I've heard from some experts that it's either sometimes or occasionally.

Depends. Roughly depends on number of links to a particular page and main domain's PR. And of course how often content get updated.

So, it varies from 2-3 days to 2-3 months ...

so as i understand if i will add more incoming links from new sites and update existing - they will be updated at google after some time.

well, let see.

yes, you will notice frequent crawling. By the way, concentrate on getting more link love to main domain.