Not sure what you mean by PR updates criteria but there was a minor PR update just a few days go. Just Keep adding quailty content building good banklinks and PR should come. Thanks
If you are asking about time - recently If you are asking about the criteria - No one could tell you exact but only "old assumption" is mainly based number of number of backlinks pointing to your website and each of its PR contained, this the number of outbound link on each links page that has link pointing to you. The more outbound links = the less for each website would get for PR.
We have the same thought Zoom As long as you can gather quality backlinks Google will recognize your effort. And by the way about the PR update, I don't know if this is accurate or not but I just happen to see this Next PR Update while browsing on the net.
only new thing added up as we know is speed. other then that content, backlink... etc plays their role as usual.
Google normally update their page rank after every 3 months. But they never disclose that fact. I have asked several time to mattcutt but he didn't tell me. You may get good rank if your site have quality backlinks. You may comments dofollow blog, guest post on other blog which have good page rank their you are allow to put your link. Forum signature also help because most of the people use their link as signature. You also may submit article to do follow directory like ezinearticles and submit your blog to blog directories Thanks