That's the most hated question on DP. Always do a search because you'll find hundreds of existing threads. No one ever knows when the PR will update. Google has no set schedule - they do it when they want.
Guys, things changed. As we all know, Google update PR about every 3 months. But this time, the update came earlier than usual. So the question is Google will still update at the end of this month or beginning of next?
I think you must not focus on PR try to focus more on your SERP because it will bring you traffic than your PR.
Google thinks they are god for some reason and now do things when thy see fit. Generally it was to be roughly 3 month intervals, but again they are god so whatever they want, when they see fit they do so. There is usually no warning or very little aside from a few fluctuations and talk we sometimes get a feel for a warning. I think they just flip the switch whenever they want. Who knows really, but the general thought is 3-4 month intervals with 6 months max on some rare occasions I think I seen posted or mentioned in the past.
I can't find the link but there was a website to estimate the next PR update according to stats of previous wait between PR updates