All models are charged based on usage tokens. Once you get started with the API, head on over to Developer Platform’s Billing to add in your payment method. This is also where you can see your API usage, which we update in near-realtime. Once your credits are over, you will be billed once a month. More details on payment will be outlined in your contract.
At this moment we don’t have support to add API limits directly in the dashboard, make sure you use the API in your code responsibly.