I hate it when people ask this question...
The only people who truly know when the next update is going to be are the spoofs at Google. Historically,
visible PageRank updates occur approximately every 3 months (that's the PR in your google toolbar, which means we should expect an update in April. However, Google has gotten much less predictable about this, so who knows.
Now real PR is always updated and continually factored into search results. By the time you see a change in your toolbar, any effects of that change has already been factored into your search results.
SEO is something I actually like more than design (bows head down in shame)
Check out something Matt mentioned in one of his blog posts
here. to understand PR updates more :)