Google PR can be achieved anywhere from 7 days to 8 months -depending on the time of PR export (from the time of snapshot) to the toolbar, indexing of the site and inbound links.
For example --
I have 2 PR4 sites --
I open a site-- and put unique contents on it which are already there-
Link it from your 2 PR4 sites
It gets indexed in 12 hrs --including inbound links
Google takes PR snapshot 18 hrs from the launch of your site --
It exports the PR to the toolbar the next day--
VOILA --
You get PR for your new site in 36 hrs.
Although it is higly unlikely -in practical scenario-- but thats the way it works.