So glad I found this as I was fretting a little about the PR of one of my sites... now I can relax and just concentrate on adding new content.
Getting good PR is not depend on time. If you are able to earn more high pr links to your site and these links are more relevant to your site then you can get PR 6 in just few months. As google update PR in each 3 months so we can say that 3 months are enough to get 6 pr with good work.