make an update to an object or even Google is not easy to do because it should be studied first before it can be done to beautiful things or making it easy to update a Google.
It is supposed to be every 3mths but the big G haven't update the toolbar PR for close to 5mths now if I'm not mistaken. If your site is conversion driven, then I'd say pay more attention to your search rankings rather than cosmetic stuff like PR.