I have an annual plan, so how will a regular monthly credit purchase work?

Your regular monthly credit purchase payment will be taken on the same date every month as your subscription billing date, and credits and credit boosts will be automatically added if your card payment is successful. ​ 

So, for example, if your billing date is the 16th March annually, then your regular monthly credit purchase would be processed on the 16th of every month. 

See more Recurring Monthly Credit Purchase FAQs.