Look in Global.asax you will see this:

 ClassifiedsHttpApplication cha = new ClassifiedsHttpApplication();

It will cause the following code to execute(A timer is cretaed)

 public void Application_Start(Object sender, EventArgs e)
        {
            _siteUrl = GetSiteUrl();

            // set-up Settings
            ClassifiedsHttpApplication.ClassifiedsApplicationSettings = SiteSettings.LoadFromConfiguration();

            // set-up maintenance timer
            _lastNotificationTime = DateTime.Now;
            TimerCallback callback = new TimerCallback(Maintenance.HourlyMaintenanceTimer);
            Timer _hourlyTimer = new Timer(callback, null, TimeSpan.Zero, TimeSpan.FromHours(1.0));
        }

ClassifiedsHttpApplication derives from  HttpApplication .

The codes there runs as long the web application is alive.

Thanks
/Kadji