The Checkout hosted pages by default does not have an option to display the billing period.
However, as an alternative you can set this by going to Settings>> Customer-facing essentials>>Checkout & Self-serve portal >> Info and Alerts. Towards the end of the webpage under the Checkout section, go to the " Trial information " section.
Here you can specify each plan using the mergevar below:
{{#plan.id= < Enter Pland id here>}}
This is an annual plan
{{/plan.id= <Enter plan id here>}}
Click the "Update" button
Refer the screenshots below:
Here is the screenshot of how the checkout would reflect the text you put in:
Would this work for you?
James Rose
Please see below. This really should say if it is monthly, yearly, etc. It only says Annual because that's the name of the plan