var timerInterval = 60 * 10 * 1000; // 10 min
var sessionTimer = setTimeout('sessionPing()', timerInterval);

function sessionPing() {		
    var myAjax = new Ajax('/session_util/session_ping.jsp?f='+ Math.random(), {method: 'get'});
	myAjax.request();
	sessionTimer = setTimeout('sessionPing()', timerInterval);
}