In the case of the United States Postal Service's First Class Mail, it is generally for a period of one year.
Companies operating in the UK outside London are required either to be licensed by HMRC or supervised by an authorised body, such as the Financial Conduct Authority.
When new users apply for the service, the company will assign them an address where they can receive their US mail and packages.
This type of service is mostly used by people moving outside the US, such as expatriates, long term travelers or retirees and international shoppers who wish to shop online for US products.
The customer usually has the option to instruct the mail forwarding company how, when and where to ship their package and also to pick which shipper to use, e.g. FedEx, DHL, UPS or even USPS.