Every 30 days from the initial purchase date! If you purchased an annual subscription, you will be billed 365 days from the initial purchase date. If you purchased the Black Friday promotion, your next year will be full price.
(Note: This does not mean once a month, it means as stated above, EVERY 30 DAYS. Due to the current calendar year - it is possible to be sometimes be billed twice in a larger month than when you originally signed up. For example, a person who signed up February 29th on a leap year will be charged twice in March the following year.)