Large projects are paid on actual documented costs (except capped projects). You must provide cost documentation within 90 days of work completion, and the recipient must certify completion and submit the final claim within 180 days of completion or the project deadline, whichever comes first.
Source: PAPPG Table 34.
Build the closeout file as the project runs. Reconstructing invoices, change orders and procurement records years later is where large-project deobligations come from.