Your credits refresh on the day of the month that you began the subscription. So if your subscription started on March 15, your credits will be refreshed on the 15th of each month.
You can also check when your next refresh is by hovering your mouse over your initials on the lower left corner of the webapp. A small popup window will appear and provide you with the information.
If you have any trouble, please contact our Support team by clicking "Submit a request" at the top right of this page for further assistance.