Priority Mail boxes can be reused as many times as you like, as long as they are used for Priority Mail shipments. The USPS does not allow for its branded boxes to be used in the “wrong” service, so do not use Flat Rate boxes for Priority Mail services, and vice-versa.
If you try to use a branded box from USPS in the wrong service, they can return the package to you before it is delivered, or mark the box as Postage Due and charge the recipient a fee for the shipment to be finally delivered. This should be avoided as a customer does not want their package held up by the post office and require an extra shipping payment, especially if they already paid for shipping when they initially purchased your product.
Lastly, if you decide to re-use any boxes for shipments, make sure you use a marker to clear off any old bar codes and addresses from previous deliveries.