Credits are valid for the duration of your subscription period or until you use them up, whichever comes first. So if you are on a monthly subscription and you use up all of your credits before the end of the month, a new pack will automatically start on your account, ensuring no drop in service.

Credits do not roll over from one subscription period to the next. Whether you have a monthly or annual subscription, when you come to the end of that month or year, a new pack of credits will start and any unused credits will be lost.