it should be about 3 months but it not fixed tough. sometime it took longer than that. i think the longest periods was like 140 days. cant really remember it.
i thought it was just publically 3 months (ie for google toolbar) but it was changing all the time within google?
I agree with ya. Only the Big-Bad-Ol-Daddy knows when to update PR. It updated on Jan 25th 2007 and April 27th 2007. So you can calculate.
Has it ever been anything shorter than 3 months? Because if it hasn't it looks like Google does its PR update quarterly then.