Credits are issued to your account on each billing/renewal date, after you have been successfully billed. This happens on the same date as you enrollment date each month, or each year, depending on your membership plan.
For example, someone who enrolls on March 3rd for a monthly membership plan will have a renewal on the 3rd of each month. You will then receive your next credit(s) at this time.
You can access the Billing Information area of your account to view the date when the next credit(s) will be added to your account.