If you have not yet switched to a Premium plan, and have auto-subscription turned on, you will get charged when you have reached the limit of your free registrations. View the amount of free registrations you have left on your Billing page.
If you have already become a Premium subscriber, you will be charged at the end of each subscription term that you chose. Monthly will be on the day of the month you started your Premium plan, and yearly will be annually on that date.