The 60-day device inactivity period cannot be modified, but, for licensed users, you can delete endpoints manually through the Inventory or with the Inventory API and this way make the period shorter.
Unlicensed (i.e. free plan) users must wait for 60 days before licenses are released.
If the device comes back online with ABR still installed, it will reappear in the inventory and the 60-day inactivity period will reset.