What is the best frequency to visit your suppliers? Annual, Semiannual or Quarterly?

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.