Depending on your subscription, you will receive a given number of credits each month. At each monthly renewal all unused credits are lost and a fresh allotment of credits is credited to your account.
In addition and at any time you can purchase top-up credits. These credits have no expiry date. To purchase a Top-up, simply click on the Payments icon:
Now click on the Topup button.
You will have a series of options to choose from, depending on your subscription edition (credits cost more for the Free Edition).