$(document).ready(function(){

   idleTime = 0;

   //Increment the idle time counter every second.
   var idleInterval = setInterval("timerIncrement()", 1000);

   function timerIncrement()
   {
     idleTime++;
     if (idleTime > 2)
     {
       doPreload();
     }
   }

   //Zero the idle timer on mouse movement.
   $(this).mousemove(function(e){
      idleTime = 0;
   });

   function doPreload()
   {
     //Preload images, etc.
   }

})

<script type="text/javascript">
idleTime = 0;
$(document).ready(function () {
    //Increment the idle time counter every minute.
    var idleInterval = setInterval("timerIncrement()", 60000); // 1 minute

    //Zero the idle timer on mouse movement.
    $(this).mousemove(function (e) {
        idleTime = 0;
    });
    $(this).keypress(function (e) {
        idleTime = 0;
    });
})
function timerIncrement() {
    idleTime = idleTime + 1;
    if (idleTime > 19) { // 20 minutes
        window.location.reload();
    }
}
</script>

//when the document is loaded
$(function(){

  //create an event handler for the mousemove
  var preLoadTimer;
  $(this).mousemove(function(e){
    //clear prior timeout, if any
    window.clearTimeout(preLoadTimer);

    //create new timeout.
    preLoadTimer = window.setTimeout(doPreLoad, 2000);
  });

});

// Call aFunction after 1 second
window.setTimeout(aFunction, 1000);