All plans, including the freeTrial plan, are limited based on usage. Usage is measured in tokens and is measured on a monthly basis.
You can see the details of your current plan, who the plan is licensed to, and your token usage in the current billing cycle by going to the Extensions > SheetGPT > Update License menu item in any Google Sheet where you are using SheetGPT:
This will fetch the latest plan details from the SheetGPT backend and show you your current cycle's usage:
You can always upgrade, downgrade or cancel your SheetGPT plan, or contact support if you are having issues.