The data starts collecting as soon as you receive your first new visitor after changing your code.

I should add that having both versions of the same Flag Counter code on your site at the same time will not be recommended, and will have unpredictable results.

You can see an example of what it looks like by just going to the home page (www.flagcounter.com) and choosing "recent new countries" instead of "top countries".  As you customize the look and feel of the widget, you can see how the results will look based on the visitor data of www.flagcounter.com itself.

What I would like to see is a widget in which the flags are arranged by order of number of visitors (as in the "top countries" choice), but instead of number of visitors, I want to see how long ago someone from that country visited my site.  That way, I will not have the mess of duplicate flags I now see when I choose "recent new countries".

Thank you Jeremy!! This is a very cool option/upgrade of the flagcounter.
PS. I see a bug where the '10 hours ago' is shown as '1 hour ago', but overall this is a very great option, thanks a lot

@Pillandia: Yes it is possible, just regenerate your HTML and pick the option of 'recent visitors'

Now I know how I get it.