But if the question is more, "When will the improvements I made show up in my SERP rankings" it varies.
Depending on what changes you are making, you can use Google Webmaster to see when it picks up on it (indexes the pages, recrawls, finds the new links, etc.) to get some visibility and then check your SERP to see how it turned out.
Changing your content reguarly and having backlinks will make the robots come back more often as they are expecting change. I think there is a setting in webmaster tools to change the frequency they visit but I have always left this on the defaults.