Sometimes Google will update Page rank monthly. Sometimes it will take 2-3 months for PR Updation. If you have launched a new website you will have to wait for next Google updation. By this time apply effective link building techniques for your site. Google give importance to the number of back links to a site when calculating Page rank.
Google PR updates are done in every 3 month so i think if you want PR for your site than you have to wait for 3 month atleast and also you need to generate good quality backlink for your site.
While PR is updated roughly ever three months sometimes you'll see chnages earlier for a new site because you could get your site indexed right before they do the big "snapshot" of the web before they export/publish the data. Sometimes new site will show PR after 4-8 weeks if they are lucky. Its timing sometimes.
Some websites are lucky and get PR within a few days. Its all a matter of when google wants to do it. Google also gives some sites (mostly larger businesses) PR at off times. Although some get it quicker!