Last year I kept records for recalculations of Page Rank and I saw first hand that it does it every 26 days (plus/minus)
Now I realized something else. That the calculation is done during the full month, not like a full radiography in the last days.
For example: I have a site (A) that I had a link towards it from another site of mine(B), with PR4. 3 weeks ago I took the link out from B and after 1 week time I reposted the link. Meanwhile, Google recalculated the page rank and the target site lost 1 point of page rank + google don’t see the incoming links from that original site, just because when he crawled my site, it did not found the link there. I hope that in the next re crawling it will come back to original status.