<asp:ScriptManager ID="ScriptManager1" runat="server" EnableScriptGlobalization="True" EnableScriptLocalization="True">
<Services>
<asp:ServiceReference Path="~/WebServices/KeepSessionAlive.asmx" />
</Services>
</asp:ScriptManager>

<script type="text/javascript" language="javascript"> 
function KeepSessionAlive() { 
    setInterval( 'doKeepSessionAlive()', 1000*5 );
} 
     
function doKeepSessionAlive() 
{
   var d = new Date();
   MyWebApplication.WebServices.KeepSessionAlive.CallingTime(); 
}        
Sys.Application.add_load(KeepSessionAlive); 
</script>

 public class KeepSessionAlive : System.Web.Services.WebService
 {
        public KeepSessionAlive() { }

        [WebMethod(EnableSession = true)]
        public void CallingTime() 
        {
           Session["KeepSessionAlive_CallingTime"] = DateTime.Now; 
        }
    }