Check for idle status in meteor by using 'setInterval'
I think interval is overkill why not something like this:

var timer;
Template.example.onRendered(function() {
  timer = setTimeout(showModal, 60000* 15);
});

Template.example.events({
    'mousemove, mousedown, touchstart, keypress': function() {
       clearTimeout(timer);
       timer = setTimeout(showModal, 60000* 15);
    }
});
showModal = function(){};


Edit:

You just want to wait for 15 minutes so set a time out for that time and eventually reset timer. Create a timer variable that will hold the timer id so you can reset (delete&amp;recreate) when a user do something.