G Suite invoices are sent to customer no later than 15th day of the following month. The services are billed retroactively - for the previous month.
In the case of the annual tariff with an annual payment you will be always contacted before the start of the next annual period. You will receive next annual invoice and information about the approaching end of your annual subscription. This is the moment when you can reduce the number of tariffs or to switch to the flexible tariff.