By default, LANDesk keeps the inventory information forever but this can be configured.
Go to Configure - Services in your Management Console on your Core Server and look at the inventory settings. You should see an option similar to 'Days to keep inventory scans' (it changes a little depending on the version). Set this to the value you want and it will delete machines that have not returned a scan after the set number of days.
Be careful that you know your systems are configured to send their scans properly otherwise you might delete machines that are still active.
I quite like irecall from networkd, it has an archiving feature that makes sure you don't lose important information when you set this value.
Mark Star - http://www.marxtar.com
Home of Power State Notifier & Wake-On-WAN for LANDesk