How to inform the user that the session is going to end in next few minutes due to inactivity.

Write code in masterpage.
protected override void OnInit(EventArgs e)
{
if (!(Request.Url.ToString().IndexOf("Login.aspx") > 0) && InternalSession.InternalUserInformation!=null)
{
base.OnInit(e);
string script = "window.setTimeout(\"SessionEnd('" + ResolveClientUrl("~/Logout.aspx") + "');\"," + (Session.Timeout - 1) * 60000 + ");";
ScriptManager.RegisterStartupScript(this.Page, this.GetType(), "SessionTimeout", script, true);
}
}


Js file.
In the Javascript file put the below code.
function SessionEnd(url) {
alert('**ATTENTION** There has been no activity for the last 15 minutes Your session will expire in 5 minutes. If you perform no action before it expires, you will be required to log in again');
}