How often google updates

I have couple of sites which PR should get higher, but it's alraedy a month I see no change.
So I was curious is there any certain time periods between Google updates or are there any signs whether google is going to update in near times.

 

visit this:
http://www.seocompany.ca/pagerank/page-rank-update-list.html

 

Thanks, that at least gave me some oriantation!

 

This is how you get your pr updated:

http://www.aojon.com/want-to-get-a-pagerank-of-10-in-3-months-for-free-heres-how/

 

And you can check PR here :
http://livepr.raketforskning.com/

 

Matt Cutts provides an excellent explanation about updates that may help you.

 

Thanks for links guys.