All users are billed on their Billing Anniversary Date (the date you originally signed up for service)
Users on a monthly subscription plan will be pre-pay monthly. This bill will also include any applicable overages for the previous month. If you're an annual subscriber, you will only receive a bill before the end of the term if you've accrued any overages.
Pay as you Go/Transactional
Pay as you Go users will be billed for usage charges based on the previous months usage.