jQuery keyhold plugin. Records the number of milliseconds a key is pressed.

jquery.keyhold.js

(function($) {

  var k = {};
    
  $(window).bind('keydown', function(e) {
      if (!k[e.which]) {
           k[e.which] = +new Date;
      }
  });
   
  $(window).bind('keyup', function(e) {
      e.duration = new Date - k[e.which];
      e.type = 'keyhold';
      delete k[e.which];
      $(e.target).trigger(e);
  });

})(jQuery);

jquery.keyhold.min.js

(function(a){var b={};a(window).bind("keydown",function(a){b[a.which]||(b[a.which]=+(new Date))}),a(window).bind("keyup",function(c){c.duration=new Date-b[c.which],c.type="keyhold",delete b[c.which],a(c.target).trigger(c)})})(jQuery)