AWS D1.1 is a structural welding code. It has instructions for testing, and acceptance norms for various welds. Similar codes like ASME Sec IX also require welds to be tested during manufacture only. In other words a weld is checked only once prior to field use.No test as far as I am aware is stipulated after the structure/equipment is commissioned.
If by frequency you mean the extent of testing e.g. 10%, 25%, 100%etc. then the answer is that the extent of testing is as per specifications or as per the code governing manufacture of the particular item under consideration like code for cylindrical vessels, pressure vessels etc.
I hope that my answer is clear and is of help.
Regards and best of luck.
V.V.Vipin