I'll find this out and let you know the exact % and how many get it. What we do is control who gets the first 4 and they will tend to get it the entire fight (unless people die and get rez'd, hint!), but at some point when others proc it, it becomes an issue of people realizing they have it and know where to move so it does not spread. So as the fight progresses, it becomes a matter of player responsibility, if they have Heart of Fire, they need to move to a safe spot and not spread it needlessly, same goes for those without HoF, they need to avoid those with it so they also don't get it.
FYI at the end of the fight, everyone gets Heart of Fire!