I think that pagerank is updated dynamically as soon as a site gets new links, its just visually the google toolbar updates to the actually pagerank every few months. But as long as you are getting more links your rankings will increase, no need to wait for google toolbar pr updates to see results from my experience.
First you should clarify which PR you are referring to. There are two types of Google PR, one internal and on external. Internal is for search rankings and is updated regularly, daily. External is for the Google Toolbar and has been historically updated about every three months, though it has been known to update at random times also (anywhere from twice in one month, to once every four or five months). Here's a nice snapshot of external PR updates historically: seocompany.ca/pagerank/page-rank-update-list.html *Would post a live link but am unable to until I get reputation here. Hope that helps!
PR updates are going to be the main concern for the folks who are interested in buying and selling links and no wonder the majority of professionals have completely forgotten about checking their sites' green bar values because it has been seen that a relevant PR zero link can be much better than an irrelevant PR 3 vote.