How to redirect the page in 20min if no actitvity happens on it

You can use javascript setTimeout for calling a function after 20 min and you can write your code like the following. 20000 means 20 minutes and it will be in millisecond. Or you can use javascript
setInterval for calling a function after 20 min. You can add this setinterval and settimeout function in document.ready function of jquery or yoou can use it anywhere.

setTimeout("MethodToCall()", 20000);

window.setInterval(MethodToCall, 20000);

function MethodToCall()
{
...
//
..
}

And check the idele time like the following

var delay = 30; //seconds
    var userTimeExpired;
    var IDLE_TIMEOUT = 30; //seconds
    var idleSeconds = 0;
    document.onclick = function () {
        idleSeconds = 0;
    };
    document.onmousemove = function () {
        idleSeconds = 0;
    };
    document.onkeypress = function () {
        idleSeconds = 0;
    };
    window.setInterval(CheckIdleTime, 1000);

    function CheckIdleTime() {

        _idleSecondsCounter++;
        var popUpPanel = document.getElementById("scanCardchild");
        if (idleSeconds >= IDLE_TIMEOUT) {
           //Postback to new page here
        }
    }