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Preferably Quarterly if not Half yearly review is must.
Its depends on the products they are handled. If they produce long lead/critical/engineered materials Quaterly visit is better. Otherwise half yearly performance evaluation is enough in my view.
I suggest the visit to suppliers should be on Quarterly basis and also depending upon the Categories.
A visit may be necessary even though not planned. Major Suppliers (80/20) should be visited once a year by you or a subcontracted auditing firm of your choice. However, if there is a new persistent problem and the Supplier hasn’t corrected it, a visit may be necessary even though not planned.
This is informed by the categorisation if such suppliers - strategic, ad-hoc, seasonal or sole suppliers for example.
This greater the classification and importance to one's business the greater frequency of visits and engagements are required, in my view.