Google Page Rank

How often is the google page rank updated? does any one of you know about it? If yes then do comment on this post so that I get to know of it. Also, I have been trying my best to write quality posts and also generating some decent back links but I still fail to get a page rank while I see many sites similar to mine having a page rank of 3 and 4. I am not sure why mine has not gotten one yet. I am just hoping to get a number in my PR tool bar. For all those that have gained a very good PR, please comment on my post to let me know how you managed to do so. I can follow your trails. Thanks much!