You should consider the requirements in 9.2 Internal audit, where 9.2.2 states
a) plan, establish, implement and maintain an audit programme(s) including the frequency, methods, responsibilities, planning requirements and reporting, which shall take into consideration the importance of the processes concerned, changes affecting the organization, and the results of
previous audits;
and FAQ 20, which gives the guidance that the audit frequency must be determined based on risk and product complexity.
As already stated, you should also consider the method used, and some customers have their own specific requirements (GM - 'quality-focused checks on each shift', VW - product audits to VDA 6.5, product audit every 12 months for serial parts', FCA - audit schedule to cover all the codes/product families within a period of a calendar year' etc.