Edit in JSFiddle

var progress = setInterval(function() {
    var $bar = $('.bar');
    
    if ($bar.width()==400) {
        clearInterval(progress);
        $('.progress').removeClass('active');
    } else {
        $bar.width($bar.width()+40);
    }
    $bar.text($bar.width()/4 + "%");
}, 800);

<div class="container">
    <div class="progress progress-striped active">
        <div class="bar" style="width: 0%;"></div>
    </div>
</div>

@import url('http://twitter.github.com/bootstrap/assets/css/bootstrap.css');

.container {
    margin-top: 30px;
    width: 400px;
}