Rather than timing the custom item via MediaLab, try adding this javascript to your custom item so that the continue/submit button will be disabled for whatever amount of time you like. In this sample script the button in the HTML form will be disabled for the first 10 seconds.
<script language="javaScript">
<!--
continue_at = "10000";
function continue_timer() { self.setTimeout("continue_enable();",continue_at); }
function continue_enable()
{
var submitObj = document.getElementById('continue_button');
submitObj.disabled = false;
}
//-->
</script>
<body onLoad="continue_timer();">
<form method="post">
<p>Your Answer: <input type="text" name="var" size="20"></p>
<p><input name="continue_button" type="submit" value="Continue" disabled></p>
</form>
</body>
