Free apps get killed every 24 hours and wont start up again until they get a hit (http://www.mail-archive.com/heroku@googlegroups.com/msg04562...). Possibly there are some paid apps that aren't getting used, but why would you pay every month for an app no one uses?
Probably > 95% of them are free apps with hardly any traffic.
But this is part of the beauty of Heroku. If you hack something together instead just storing it with "git push origin" you can do "git push heroku" and actually demo it to the world for free.
I like to think of Heroku's 100,000 apps as public demos of a lot of GitHub repositories.
Interesting, I totally thought they would have hit this a while ago. For some reason I assumed they might have hit a magnitude higher. But I guess I totally misunderstood their popularity. Whoo echo chamber.
So, the answer is: probably close to zero.