We offer two billing cycles - monthly and yearly.
When you upgrade to a paid account with monthly billing, you'll be billed immediately for the next calendar month. Your payments will then recur automatically every month for as long as you are signed up to that account. You can downgrade at any time.
Similarly, when you upgrade to a paid account with annual billing, you will be billed for the next year. And if you choose to upgrade to annual billing when on a monthly plan (you'll save 15%!) any credit on your account will could towards your bill.