goodroi - 12:18 pm on Oct 12, 2012 (gmt 0)
Many websites can see a change in rankings within 1 day of making an improvement to their content. Of course this depends on a few factors that are unique to each situation.
How often is Google crawling your website (they need to crawl it to notice the change). Websites with more backlinks and a pattern of regularly updating content tend to be crawled more often.
Are you trying to boost rankings or reverse a penalty? If you are looking to move up a few spots in the rankings, then you can see a boost as soon as Google revisits your site. If you have been penalized then you may need to wait for your penalty to expire for any ranking change to occur or for a Googler to manually review a formal reconsideration request.
I would also suggest that you try to keep your changes small and simple. This will make it easier for you to figure out what changes help you and what changes might actually end up hurting you. If you have many changes to make, try to break them up into smaller changes. Otherwise you might end up adding some good things and some bad things and not being able to figure out which is which. Good luck.