Your billing cycle is set based on your plan's date of purchase.
For example, a monthly plan purchased on the 5th of January will reset on the 5th of each following month.
When subscribed to a Monthly plan, your pool of credits will include credits for the current month, as well as unused credits from previous months.
Unused credits will be rolled over to a maximum quota of your monthly credits X 12.
For example, if you get 120 credits per month, the maximum number of credits you can have is 1,440 (120*12 months).
When subscribed to an Annual plan, your pool of credits will reset at the beginning of every annual billing cycle.
Please note that credits cannot be rolled over on a Free plan. Furthermore, if you cancel your paid subscription, the unused credits will be expired at the end of the last paid cycle.