nah, it is not a set thing, they've had 106 days between previous updates. i think they want to have at leat 4 a year, but they are not bang on when it comes to dividing the year into quarers.
What I really wonder is that, how they determine the PR.
I know the number of backlinks, but what about the backlinking site and the ones before that and before that?
No one can really tell though.
They change it so often that its hard to keep up.
Just make a good site, link it properly and whatever they do you are safe.
Let me clarify this..
I mean that
It depends on the site's PR that points to you
For ex. if a PR 5 site is pointing to you can probaly have a pr 3 or 4 the next update. if you maintain to link with this high kind PR sites