There are currently no option/attribute of duration, which will display the duration in the form in which you need. I had the same requirement and wrote small function, which use DOM manipulations to hide unneeded input fields. To de mostly safe I initialized doc, win and table variables first of all by the following code:

var doc = g_form.getControl("someElementFromTheForm").ownerDocument,
	win = doc.defaultView,
	table = g_form.getTableName();

where someElementFromTheForm is the column name of some element of the form. The main code is the following:

var hideMinSecFields = function (field) {
		function getElem (field, suffix, prefix) {
			return doc.getElementById((prefix || "") + table + "." + field + (suffix || ""));
		}
		var controlHour = getElem(field, "dur_hour", "ni."),
			controlMin = getElem(field, "dur_min", "ni."),
			controlSec = getElem(field, "dur_sec", "ni."), cs;			

		if (controlMin != null && controlMin.style != null) {
			controlMin.style.display = "none";
		}
		if (controlSec != null && controlSec.style != null) {
			if (win != null && win.getComputedStyle != null) {
				cs = win.getComputedStyle(controlSec, null);
				if (controlHour != null && controlHour.style != null && cs != null) {
					controlHour.style.borderRightStyle = cs.getPropertyValue("border-right-style");
					controlHour.style.borderRightWidth = cs.getPropertyValue("border-right-width");
					controlHour.style.borderRightColor = cs.getPropertyValue("border-right-color");
					controlHour.style.borderBottomRightRadius = cs.getPropertyValue("border-bottom-right-radius");
					controlHour.style.borderTopRightRadius = cs.getPropertyValue("border-top-right-radius");
				}
			}
			controlSec.style.display = "none";
		}
	};

To hide the min/sec fields you need just call hideMinSecFields function with the name of the duration field as parameter.

How do I run this? onLoad client script?

Yes of cause. It's a part of code, which will be start onLoad.

Sorry, can you give me the steps? I'm not sure where and how to implement this.

I'm currently in business trip, but I'll try to post at the weekend more full example of the code or small demo application on GitHub, which uses the code, which I posted.