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.
When subscribed to an annual plan, your pool of credits will reset at the beginning of every annual billing cycle.