Google update prs each 3 months cycle per my recent knowledge , but google does this randomly some times.
Google doesnt have a systematic PR update. meaning that they dont update PR every 3 months, 2 months 6 months etc. it is random and comes out of nowhere. i've seen a PR update then 8 months later another then 3 months later another. there is no way to predict a PR update.
No one can tell you about updating of algorithms. You should stay in touch with Google Webmaster Guidelines cause here is the place that you can get anything of SEO confusions, questions and answers. The main thing is Google is changing daily to avoid web spam.