It usually takes at least three full months for all of your inbound links to be calculated and counted as back links. Google's Page Rank is calculated based on the relevancy, quantity and quality of your inbound links. But ranking on Google also depends of how old is domain.
actually, inbound links are calculated constantly and are counted as back links. real pagerank is also calculated on an ongoing basis. here's more about what pagerank is and isn't and some of its other myths and misunderstandings.
I think that any web site have PR just born on google. and have it changing constantly but we see the changes for the toolbar PR only. So do nor worry - everyweb site have PR. In most cases it is not shown till PR update