[Sloerie]

Hi there Happysmut,
from what I know (but things change all the time at google nowadays) PR is calculated internally all the time and can be different from what you can see in the googlebar. So effectively your actual PR can change anytime (and so does your position in google).
But you can get an idea on how frequent updates are done for your specific site by estimating how much you are visited by googlebot, googles spider.

If your site seems "interesting" to google you will get more visits and I believe that your PR gets recalculated more oftn as well.

But as you know, nobody outside google really knows...

Good luck with your SEO efforts!