The frequency of billing for a subscription is determined when you set up the application. You can select either monthly or yearly frequency.
You can view the payment frequency of an existing subscription application in Casting Settings.
View Payment Frequency
1. Click your avatar on the bottom-left to open the User menu.
2. Select "Casting Settings".
Click on the title of the Application to open it. 
Scroll down to the "Payments Page" section. Click the downward arrow to expand it.
Under "Interval" you can see whether the payments are monthly or yearly.
Note, once recurring payments have been set up and saved you cannot edit the amount or frequency.
For detailed information about payments see the Casting Payments Help Guide.