This approach will not work because every time your page loads the Session slides and the timeout is reset.
For example, if your session is set to 30 minutes and someone firsts loads the page then in 30 minutes the session will timeout if there is no activity. However, for your C# code to run the page must postback and if that happens after 10 minutes the session is reset and will timeout in 30 minutes, not in 20 minutes.
The way to do this is by using setTimeout() in JavaScript to run once a minute and at each minute the function will decrement 1. This is not a guarantee because you are not contacting the server to get the true timeout, but it is a crude attempt at doing something that technically isn't possible.
So, you could do something like this (just a simple example that you'll need to finish):
$(document).ready(function () {
SessionTimeout(30);
});
function SessionTimeout(var minutesLeft)
{
$("#lblTimeout").val(minutesLeft);
setTimeout(function(){SessionTimeout(minutesLeft--);}, 1000);
}