Parts Inventory needs to be counted annually…at a bare minimum.

Typically, you’d want to count it just before your year-end, after a physical inventory count. You’ll need to do Inventory Reconciliation, where Parts Inventory is itemized line by line (Dollars On-Hand, Work In Process, etc), and then matched next to Accounting Parts Inventory line by line (General Ledger Inventory Value, Negative On-hands, Unposted Parts Inventory, etc).