The purpose of Retainage is to hold back money from a contractor for incomplete or deficient work. Ten percent (10%) appears to be the norm across the United States including Hawaii.
Retainage is held by prime contractors against subcontractors and/or by owners against prime contractors.
There is little or no reason to hold the Retainage once the work has been completed and accepted in accordance with the contract documents.
One technique I have successfully used is to include wording in the contract documents that “The contractor shall notify the owner in writing that all punch list inspection items are complete”.
Here is a definition of a Punch List found in my Glossary "A list prepared by the owner or his/her authorized representative of items of work requiring immediate corrective or completion action by the contractor."
As the prime contractor or owner’s representative I would not hold or attend a punch list re-inspection for the project until the aforementioned notification has been received.
This practice tends to minimize, if not eliminates, incomplete punch list re-inspections and expedites the Retainage payment to the contractor because when all punch list items have been completed and accepted in accordance with the contract documents the contractor is entitled to submit a payment request for the Retainage amount. And said payment should not be unreasonably withheld. Usually contract documents include payment processing requirements such as “payments are due within 30 days of receipt, etc.”
This practice also tends to be fair to both owner and contractor.