Monthly subscriptions are just that, ongoing subscriptions where you will be charged monthly until you cancel the subscription.
If you sign up on the 15th of the month, you'll be billed on the 15th of each subsequent month.
You can cancel at any time by visiting your Account Settings page and hitting the big read "Cancel Subscription" button.
You can also change your mind about the plan you want and switch to other plans by using the green "switch to" buttons. Doing so will switch your plan right away and will automatically pro-rate the next bill.
You can update your card information, see when and how much you'll be billed next, and request any receipts on the right hand side under the heading "Change Card Info."
