Hi Patrik:

You can do this using the Subscription app and setting up your product as a Subscription product. This app is available in the Enterprise Edition.

You can find more details about the Subscription app here: Subscriptions — Odoo 15.0 documentation

Hi 

Goto Other Info tab of your Bill / Invoice. 

then click on Auto Post. ( image is shown below for your ease. ) 

You can select Monthly , Quarterly, Yearly or specific dates.

You can do this for the both Bills and Invoices  

Thank you Paresh, much appretiated

Hi,

Try the following steps.

• Go to the Accounting module in Odoo and click on "Recurring Documents" in the left-hand menu.
• Click on "Create" to create a new recurring document.
• Select "Customer Invoice" as the document type.
• Fill in the details of the invoice, including the customer, product, amount, and tax code.
• In the "Recurrence" section, set the frequency to "Monthly" and specify the number of months for which you want to generate the invoice.
• Choose the start date for the recurring invoice.
• Save the recurring document.

Regards