Upgradation of Page Rank.
Page Rank updates itself in every 2-3 months. Although, real PR updates every single minute but PR Tool updates in a interval of 2-3months. The reality is the above mentioned facts are based on personal experiences but Google did not reveal anything regarding PR upgradation. It is not transparent and Google may upgrade PR according to its whims and fancies. It may take few days or many odd months. We never know! :)