How to get info on what key was pressed on for how long?

Here you go:

var pressed = {};

window.onkeydown = function(e) {
    if ( pressed[e.which] ) return;
    pressed[e.which] = e.timeStamp;
};

window.onkeyup = function(e) {
    if ( !pressed[e.which] ) return;
    var duration = ( e.timeStamp - pressed[e.which] ) / 1000;
    // Key "e.which" was pressed for "duration" seconds
    pressed[e.which] = 0;
};

Live demo: http://jsfiddle.net/EeXVX/1/show/

(remove the "show/" part of the URL to view the code for the demo)

So you have the pressed object which monitors which keys are currently being pressed and at what point (in time) they have been pressed.

Inside the keyup handler, you determine if the key was being pressed, and if so, calculate the duration by subtracting the time-stamps of the keyup/keydown events.

Have you tried doing somethign like,

1. onkeydown, remove keydown listener.
2. onkeyup, attach keydown listener again and computer the time?