Billing is immediate, but there is a 9-day trial period where if you decide to cancel you will receive a full refund. You can also of course cancel your membership at any time from your Members management page. There are no refunds for memberships past the 9-day trial period, and there is no partial or pro-rated refunding.
If you choose to cancel your membership it will automatically expire at the end of the billing cycle. You will still be able to access premium members-only videos and courses until the end of the billing cycle.