SERPS change every time there is need for that. When your page gets indexed, the results pages change. When someone elses page gets indexed the result change. If you are doing things right, you can see improvement in your rankings daily, even a difference between the morning and evening rankings.
Google updates it whenever it wants, but has always stressed not to worry too much on PR but to focus on quality content, and backlinks
Nothing can be predicted about it. Google has no specific time or period to increase the Page ranks. Its a continuous process, some go up and some go down at the same time.
Pretty much. Every minute of every day there's new content being published on the entire world wide web and Google bots and search engines are picking up most or all of these and deciding where they will be ranked, if at all. The best thing for any SEO to do is keep building anchor text backlinks for the keyword they want ranked. No matter how long it might take, if you're persistent in the link building process you're pretty much guaranteed to get ranked for your keyword.