I wished I knew the answer to this question. This is a question that is normally asked in webmaster’s forums. People speculate when the next update would be. The only people that knows this answer are those Google employees because they are the ones doing it.
Already, there are speculations that the next toolbar PR update would be in July because Google shows us the PR updates after 3 months. For this year 2009, the first toolbar update was on January 1st. The next update was on April 1st. I can speculate that the next update would either be on July 1st or August 1st. I chose 1st because the PR updates had occurred on the 1st day of the month. Google is not obligated to follow a schedule. It can update the toolbar PR any time.
The toolbar PR would be likely updated on July, but the real PR of web pages are updated daily. The toolbar update is lagging behind. What you see on the toolbar is what Google wants you to see. I am saying the true PR value of a site is updated daily because of my experience with an expired domain.
I bought an expired PR 3 domain in February. This was my first time of buying an expired domain. The seller was selling it for $20. I thought that was cheap because to get a permanent link from a PR 2 site, the seller might charge you $10 for 1 link. If you want to buy 2 links, that means you would have to pay $20. I saw that buying the PR 3 domain for $20 is cheap because I can put unlimited links on the site to help me make money online. I did nothing to the domain. I just installed wordpress on the domain. After 3 weeks of the purchase, I visited the site and my toolbar was showing me gray i.e. Page Rank (n/a). Google took the PR away from the site. This was before the April PR update. I thought it was a mistake, that my toolbar was acting weird. I waited for a week and when it still showed me PR n/a, I accepted the fact that the PR is gone and the site needs to be put to rest.
I also have also noticed that when I am searching for a keyword on Google, some new web pages rank higher than some web sites with PR. I wonder why would Google give that new webpage with no PR a higher search engine ranking than the already existing site with PR. Both sites have some of the keywords I am looking for. It is after a PR update, that I would notice that the new webpage would later have a higher PR than the older site.
As no one really knows when next is the toolbar PR update, you just have to continue building quality links. The more quality links you build the higher your PR would be i.e it is much easy to get your site to be PR 1, but it needs more work to get your site to a PR 3. To get your site to be PR 7, you need to know webmasters that have PR 8 sites and above to link to you. You should have a very good network to get that. Don’t get me wrong, it is possible to get a PR 7 from PR 1 sites, but you need like half a million of PR 1 sites. This is according to the tnx PR table.