Are you talking about PageRank™, Google's patented method of measuring the strength of links pointing to your pages? (i.e. the little green bar in the Google tool bar)
Or are you talking about your search engine rankings (i.e. where your pages show up in the search results)?
The PageRank on the toolbar is updated about every 3-4 months, according to Google employee Matt Cutts. But that PageRank isn't the PageRank that's actually used in Google's rankings. It's an outdated and inaccurate approximation of what your page's PageRank
might have been at some point in the past. And it's prone to glitches that send inexperienced webmasters into panic attacks ("My toolbar is greyed-out! Does this mean I've been banned?!?") when the only problem is the toolbar's having another hiccup.
That said, if you don't take it too seriously, it can be one of the general indicators of a page's overall "health" -- from the 30,000 foot view, that is. You can safely turn off the toolbar PR indicator if you want.
There's an internal PageRank number, which is what Google actually uses as but one of between 100 and 200 ranking factors. So as much press as it gets, it isn't as
all-important as some might have you believe. Important, yes. The only thing you need to be concerned about, no.
Unlike the toolbar PR, which is expressed in whole numbers between one and ten, this internal PR is a fractional number between zero and one. Nobody outside the Googleplex knows what a page's "real" PR number is, and Google employees aren't talking. They consider the actual numbers and the details of exactly how the numbers are calculated to be trade secrets.
The internal, "real" PR number is updated continuously. As they discover new links pointing to your pages, as old links disappear or as the "weight" of existing links changes, your pages' PR number is updated.
If you're talking about the rankings in the search display, those also change all the time, with the added excitement that there are multiple datacenters, each with it's own version of the Google database. And these versions are never in 100% synchronization (too much data to move around from one place to another for that to happen), and when you run a search you never know which data center(s) you're going to hit. So your search rankings can appear to jump around all over the place, when the "problem" may simply be that when you search at different times (and sometimes even when you move from one page of the results to the next) you're hitting different datacenters with different versions of the database.
As with the internal PR, your search rankings are always changing, even apart from database sync issues. They make constant changes and tweaks to the ranking algorithm and are continually discovering and indexing new pages (and the links thereon) and revisiting old pages to re-evaluate their content and links.
HTH!
--Torka
Frequency Of Page Ranking?
