private void CheckSessionTimeout()
{
    string msgSession = 'Warning: Within next 3 minutes, if you do not do anything, '+
                        ' our system will redirect to the login page. Please save changed data.';
    //time to remind, 3 minutes before session end
    int int_MilliSecondsTimeReminder = (this.Session.Timeout * 60000) - 3 * 60000;
    //time to redirect, 5 miliseconds before session end
    int int_MilliSecondsTimeOut = (this.Session.Timeout * 60000) - 5;

    string str_Script = @"
            var myTimeReminder, myTimeOut;
            clearTimeout(myTimeReminder);
            clearTimeout(myTimeOut); " +
            "var sessionTimeReminder = " + int_MilliSecondsTimeReminder.ToString() + "; " +
            "var sessionTimeout = " + int_MilliSecondsTimeOut.ToString() + ";" +
            "function doReminder(){ alert('" + msgSession + "'); }" +
            "function doRedirect(){ window.location.href='login.aspx'; }" + @"
            myTimeReminder=setTimeout('doReminder()', sessionTimeReminder);
            myTimeOut=setTimeout('doRedirect()', sessionTimeout); ";

    ScriptManager.RegisterClientScriptBlock(this.Page, this.GetType(),
          "CheckSessionOut", str_Script, true);
}       

private void Page_Load(object sender, System.EventArgs e)
{
    this.CheckSessionTimeout();
}