It’s a pretty old information. Someone, someday conducted an experiment to find the relation between your website’s search ranking on Google and the percentage of clicks or visitors that it gets.
You can keep this infographic as a quick reference to see what amount of traffic you can expect based on your search ranking on the first page of Google. So if Google Keywords Tool says there are 1,000 monthly searches for a keyword across the globe and you rank #1 all over, the site will get around 550 visitors.
Interestingly, the share of visitors is not a gradual decline. Some positions might be better than others, but get you far less traffic.