To clarify, if you change the setting in 'days to keep inventory scans' settings on the inventory service on the core, that system will be deleted from the core if it has not sent in an inventory in 'x' number of days.
We have ours at 90 days, you do not want it set too soon, especially if you have laptops that are offsite and use a Gateway / CSA since they have to be listed on the core in order to come in via the Gateway. 90 days is a good compromise.