For pay-as-you-go contracts, the very first billing period starts on the first usage of a production API key. We always bill at the end of the month, so the billing period ends on the last day of a month.
For example, John subscribes for Intento service on June 5, makes his first translation on June 8. In this case, the billing period is June 8 - June 30.
All subsequent billing periods start on the first day of the month.