If you are on a monthly subscription plan, you will receive a bill every month on your billing anniversary date.
If you are on an Annual Plan (pre-paid), you will only receive a bill on your billing anniversary date if you've incurred overages.
How will I know if I am being charged for overage?
You can review both usage and your bill by visiting the Account Management Page. Select the product you're inquiring about from the Products tab. Select Manage Plan, then View Billing Reports.