Key now needs a reference to the plugin that created it. Also added getDuration method.

src/input/keyboard/keys/Key.js

@@ -19,6 +19,7 @@ var Events = require('../events');
  * @constructor
  * @since 3.0.0
  *
+ * @param {Phaser.Input.Keyboard.KeyboardPlugin} plugin - The Keyboard Plugin instance that owns this Key object.
  * @param {integer} keyCode - The keycode of this key.
  */
 var Key = new Class({
@@ -27,10 +28,19 @@ var Key = new Class({
 
     initialize:
 
-    function Key (keyCode)
+    function Key (plugin, keyCode)
     {
         EventEmitter.call(this);
 
+        /**
+         * The Keyboard Plugin instance that owns this Key object.
+         *
+         * @name Phaser.Input.Keyboard.Key#plugin
+         * @type {Phaser.Input.Keyboard.KeyboardPlugin}
+         * @since 3.17.0
+         */
+        this.plugin = plugin;
+
         /**
          * The keycode of this key.
          *
@@ -142,6 +152,8 @@ var Key = new Class({
 
         /**
          * The number of milliseconds this key was held down for in the previous down - up sequence.
+         * This value isn't updated every game step, only when the Key changes state.
+         * To get the current duration use the `getDuration` method.
          *
          * @name Phaser.Input.Keyboard.Key#duration
          * @type {number}
@@ -339,6 +351,31 @@ var Key = new Class({
         return this;
     },
 
+    /**
+     * Returns the duration, in ms, that the Key has been held down for.
+     * 
+     * If the key is not currently down it will return zero.
+     * 
+     * The get the duration the Key was held down for in the previous up-down cycle,
+     * use the `Key.duration` property value instead.
+     *
+     * @method Phaser.Input.Keyboard.Key#getDuration
+     * @since 3.17.0
+     * 
+     * @return {integer} The duration, in ms, that the Key has been held down for if currently down.
+     */
+    getDuration: function ()
+    {
+        if (this.isDown)
+        {
+            return (this.plugin.game.loop.time - this.timeDown);
+        }
+        else
+        {
+            return 0;
+        }
+    },
+
     /**
      * Removes any bound event handlers and removes local references.
      *
@@ -350,6 +387,8 @@ var Key = new Class({
         this.removeAllListeners();
 
         this.originalEvent = null;
+
+        this.plugin = null;
     }
 
 });