1. Nobody knows precisely but it is supposed to take place once in every three months - the last one took place at the end of October so the next one should be quite soon.
2. PR does not help your SERP despite the factors improving your PR and your SERP being more or less the same (eg. link popularity).
3. PR of your site is updated on a daily basis, it's only the toolbar PR that we mortals see that is updated once every three months. So even if PR would affect your SERP you would never see a sudden SERP improvement.
hope it helps
