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?
JamesRose
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