Page Rank is done by quarter so sometime in january if the last update was in september. I don't keep track because its so over rated! you should put more effort into your serps!
No body knows for sure except for Google, try to focus more on what is good for your website visitors and traffic then PR will follow.
I think PR is updated dynamically. There is no fixed period for Pr updates. It's just like the SERPs. For example, I had a site that was switching between pr3 and pr4 several times in the matter of a few weeks.
yes, I do. if you want to sell private ads, PR is one of the factor affect the price. Besides is the number of subscribers, content, Alexa etc.