The standard simply calls for "the audit program shall be planned". Many interpret this to mean a pre-planned schedule, but you could also argue that planning could just be having an ongoing process that outputs audit activities.
If you want to go the scheduling route, how you do it is up to you. Whatever works best in the context of your organization. You can maintain such a schedule in a table, a calendar, a whiteboard,...
For example, you might just maintain a simple table with the following headings:
- Planned date
- Assigned auditor(s)
- Audit criteria and Scope
...and add rows as needed (e.g. following management reviews, CAPA actions, previous audits, etc.). Just make sure that all processes are covered at justified intervals.