Should content change after every Google Bot visit, in order to keep content fresh? If not how long? Also is there a formula for the amount of content that needs to change? 1 link? A papagraph etc?
You should change your content on a regular basis. Once a week is OK - more is better. But the most important thing is to keep at the same pace for a long period of time.
Add more content whenever you can. This is where blogs have a good advantage. Why not aim for a new article every day or at least as often as you can manage.
By "change content" we mean "add content". There is no point in shuffling the words around of existing content, unless you are rewriting for clarity or to improve the content for the user. The purpose is to gradually grow the size of your website - bigger sites generally do better than small sites because they can target a wider range of phrases (wikipedia being a prime example of this). Aside from that, existing visitors will come back if they know you are adding something new each week.
Very true. adding new content is very important both for visitors and search engines. I don't think that there is a forumla for this. Try to add something new every day, your visitors will love you and so will Google p.l.u.r.
I would change your content as often as the subject dictates. Usually, at least once a week is fine. Anything less than this is Ok, too, but any SE changes will take that much longer through each cycle. Once Google "learns" your site's update pattern, it will tend to follow that pattern itself.
As you add you are growing your site. It's good to focus on the home page, without ignoring any of the other pages.
It depends on what kind of content you write about. If you have articles about how to refinish furniture they aren't likely to change very often