It depends on a type of your membership. If you have monthly membership you will be charged upon registration and each 30 days respectively; if you signed up to one-time three-month membership; if you enrolled to one-year membership you will be billed upon registration and then after the day of the expiration of your payment that is after 365 days since the day of your registration.
Trial memberships lasts for definite period of time and are billed upon registration as well. After the term of trial membership expires the latter is automatically changes for monthly membership unless cancelled. In this case you will be billed at the day of membership upgrading and each month after it.