The trial period for Amazon Prime lasts for one full calendar month from the date the user signed up. The trial begins by entering valid payment information upon registration.
Be advised that the payment information is entered so that after the free trial period ends, Amazon can automatically begin billing the user for the service every year. In order to get the free trial period without ever having to pay, it is necessary to visit the Manage your Prime page on the Amazon website and click the do not upgrade option. This will ensure that once the free trial period ends, the user will not be billed for the Amazon Prime service unless he chooses to do so.