The two most recent PR Updates reported by our members are 20 July 2012 [webmasterworld.com] and 3 May 2012 [webmasterworld.com] Google averages about four principle PageRank updates in a year, sometimes following it with a few small tweaks and corrections. However, the pattern can be quite irregular and not too predictable. However, with under 3 months since the last one, it is far form overdue right now.
PR Updates have also become pretty much of a "non-event" among our members, because they understand that actual PR is already updated on a nearly daily basis so exporting the data to the toolbar doesn't really mean much except to those selling advertising. In those cases, a higher PR can mean a better rate for some.
I have purchased some expired domains that had been dropped. I am hoping the remaining links will still enable the domain to have page rank. Is this something that can happen and how long would it take? Is it updated immediately but not detectable by tools?
Yes, PR is updated long before the data gets exported to the toolbar. Whether your expired domains can rank based only on historical backlinks is pretty much a gamble. Sometimes yes, sometimes no. And sometimes they can inherit penalties.
Do you know how they were ranking before they expired - and did you preserve the content?