using System.Timers;
declare a timer like this
Timer timer = new Timer();
timer.Tick += new EventHandler(timer_Tick);
timer.Interval = 1000;
timer.Enabled = true;
timer.Start();
now create the event
void timer_Tick(object sender, EventArgs e)
{
if(Session["user"] == null)
{
Response.Redirect("Login.aspx");
}
}
In every 1 second your timer will tick and check for the session existence ,if it is null then it will redirect to the login page.
Set the time out for session as 10 mins.