Click Manage | Filters | Software Filters | Agent & Plug-in Filters
There are now 3 Filters which begin with "Computers with Inventory data older than" choose which one works best for you.
Also, here's the SQL to create your own report. Just edit the DATEADD field for ww or dd and the integer as approrpiate.
This is for inventory older than 1 week.
SELECT i.Guid FROM dbo.vComputer i
JOIN dbo.Inv_Inventory_Results ir ON ir._ResourceGuid = i.Guid
WHERE ir.[Collection Time] <= DATEADD (ww, -1, GETDATE ()) AND ir.Agent = 'Inventory Agent'