$(document).ready(function() {
    var n = 30;
    var tm = setInterval(countDown, 1000);

    $(".otree-btn-next").text(n);

    function countDown() {
        n--;
        $(".otree-btn-next").text(n);
        if(n == 0){
            clearInterval(tm);
            $(".otree-btn-next").prop("disabled", false);
            $(".otree-btn-next").text("Next");
        }
    }
});