It is not possible to continously update
pr in the index. You need to take a snapshot of the link structure of the whole web and then calculate
PR for all pages.
PR in your site may influence pages you have never even heard of. It´s all connected.
However, they seem to have like 10 to 15 different factors now adays that they need to snapshot update, and I have seen indications in some sites that there is actually a weekly update going on, but my best guess is that it is every time another factor that is being updated. So if
PR is one of them, then you would expect 10 to 15 weeks in between
PR updates.
I´m just speculating here,. :)