Percentages are tough because it depends on how engaging your source material is, but in my experience I have found that most people hit "x" and close the tab, rather than go back. This action is done even twice as much if the person the page is already an internet marketer because they are used to seeing another pop up ad if they hit the back button.

I think more and more websites open a new tab when clicking on a link. So hitting x to close the tab is really the only option then.

Then reason I'm asking is to get an idea of whether using a redirect plugin is even worth the effort. Some people say they get good results with it. But it only works if you hit the back button and not when closing the tab.

I rarely hit the x button or click the back button, i usually enter a new url in the browser bar.

I think it depends though, as an internet marketer you have so many things to check that there always seem to be too few browser tabs open.