[Amphiluke]

<script type="text/javascript">
window.onload = function() {
    function foo() {
        alert("Don't stop typing!");
        t = setTimeout(arguments.callee, 2000);
    }
    var t = setTimeout(foo, 2000);
    document.getElementById("my_input").onkeyup = function() {
        if (t) clearTimeout(t);
        t = setTimeout(foo, 2000);
    }
}
</script>

<input type="text" id="my_input" />