You can see the credits (characters) that you have generated and processed in a month, the current plan and much more. You can head to settings-plan for that. The monthly quota resets every month and the credits can not be accumulated or added to the next month's quota.
All generated and processed characters are counted as usage here and this number may be different from the count in documents where the only content present is counted. If there's a discrepancy, it's because some of the generated text was deleted.