Your subscription will renew on the same date each month. The date will be based on the date you purchased or changed your subscription. If you purchased or changed your subscription on the 29th, 30th or 31st day of the month, the renewal date is set to 28th.
So for example, if you purchase your subscription on January 10th, your subscription will renew on February 10th.
You can check your subscription renewal date via your developer portal.