You will automatically be charged to the card entered in your account, on a monthly recurring subscription. The month period starts on your initial purchase date which may not necessarily be the calendar month. The date of renewal can be changed at any stage by renewing early at any stage on the date you prefer.
Please note, as it's a completely automated service, the credits are added to your account at approximately the same actual time each month on the renewal date based on the time of day that the subscription was originally taken out. So if you subscribe at 5pm on the 5th, the renewal will go through and credits added at that time, each month on the 5th.