Once every 3 months it seems like but only google knows. But I would expect another one till Christmas - New Years time.
It generally takes place once evey three or four months. You could check the datacentres for any change in your page rank, for example this tool http://livepr.raketforskning.com
Matt Cutts, on his blog today, said to expect the next PR update around New Years. It is also good to note that Google is rolling out a new infrastructure right now so the newest PR values are reflected only on the upgraded data centers. Who here thinks PR means Project Runway?
That sounds about right, the seem to be sticking to a 3 month schedule at the moment. That rollout will probably show data inclusive up to the start of December though.
About every 3 months yes for the toolbar. You have to remember that Google actually updates the PR frequently, they only export it to the toolbar every 3 months or so.