Your subscription automatically renews at the end of your billing cycle. To check when your billing cycle ends, go to 'Premium Billing' found in your profile drop-down, on the top right of the website page.
On your next billing date, your subscription will be renewed, and you will be billed accordingly.
If you do not want to be charged after your subscription cycle ends, you may cancel your plan any time, by clicking 'cancel plan' in 'Premium Billing'. This will not terminate your subscription immediately, and you will avail all subscription features til the duration of the subscription cycle.
Once the subscription cycle passes, the subscription features (including credits) will be unavailable.