Javascript function not clicked whitin certain time

I need help (again). What I would like to have is a function which if there has not been clicked one time on a button in a certain time, a div will be showed. To be more clear, I'm looking for this kind of code:

    function MyFunction() {
if(not clicked one time in 8000 milliseconds)
  document.getElementById("myDiv").style.visibility = "visible";
}

Does somebody knows how I should code this? Thank you!