Though the exact length of time varies based on the package's contents and the chosen mail service, as of April 2015, USPS holds certified mail, insured parcels and ordinary parcels for 15 days after the first delivery attempt, as stated by the official USPS website. A package sent via Priority Mail Express is held for 5 days from the initial delivery attempt.
After the holding period is up, packages that do not contain perishables are returned to the sender. Packages that do contain perishables are destroyed after 10 days.
The recipient receives an initial notice and a final notice that the package is being held at the post office. Recipients can pick the item up or schedule redelivery.