Final answer:
The minimum requirement for radiography of longitudinal welds depends on the governing codes and standards, which specify the number of inches or percentage of the weld that must be examined.
Step-by-step explanation:
The minimum requirement for the extent of radiography when considering longitudinal welds is typically specified by the relevant codes and standards such as the ASME Boiler and Pressure Vessel Code or the American Welding Society (AWS) standards. However, without a specific code or standard reference in the question, we can't provide a definitive number of inches of weld length.
Generally, codes provide this kind of information, and it can vary depending on the material, type of weld, and the critical nature of the weldment. A common practice might require radiographic examination of a certain percentage of the total weld length or specific increments, such as every 6 inches, but again, this is code-dependent.