When does an inventory take place?
The Specops Inventory Client installed on a computer is listening to a Specops inventory GPO.
A full inventory occurs when the GPO is applied on a computer object for the first time. After that it does a delta inventory (delta=only the changes) every time the GPO is updated and a full inventory once every 7 days.
You can also force a full inventory on a computer by typing gpupdate /force in the command prompt.