Decided by Google... and it occurs quarterly i think. As aaron said, i think Mid January could be the next one
PR updates Everday but the Toolbar PR update occurs quarterly You can keep an eye on the PR using this PR Checking Tool - been developed by MONG
I think there use to be consitsnt time period between each toolbar PR update, but now there is no real pattern. I think this might be because they don't want PR manipulation and want to give the impression that PR is not everything.