I think you can only change it globally, for all packages on all package servers.
Settings > Notification Server > Site Server Settings > Settings > Package Service > Package Service Settings or Settings > All Settings > Notification Server > Site Server Settings > Package Service > Package Service Settings.
"Delete package files if they are unused for" 1 Year|1 Month|2 Weeks|1 Week|3 Days|2 Days|1 Day|0 Days (delete immediately)