Each plan comes with a monthly credit allowance. On your monthly renewal date your balance is reset to the full allowance for the month ahead, so plan credits are meant to be used within the month they arrive. Yearly plans work the same way: you are billed once a year and receive each month's allowance at the start of that month.
Free credits, such as check-in or referral rewards, are separate from your plan. Some carry an expiry date, shown next to each grant in Account center → Credits & orders.
If you cancel, your plan and its credits stay active until the end of the time you have paid for.