I'll jump in at this point and add my opinion to the fray. After all opinions are like anuses, everyone has one, unless you are a married woman, in which case you have two, one of which you send one of them to work.
This is where the Engineer should provide direction to the testing lab or to the CWI responsible for ensuring the testing is performed to his (the Engineer's) satisfaction.
I would categorize the welds into individual welds with lengths less than 12-inches, continuous welds that are longer than 12-inches, but less than 60-inches, and continuous welds longer than 60-inches. Then I would specify how each group should be handled. It might looks something like this:
Individual welds less than 12-inches in length: 1 in every 10 welds is tested for their full length.
Continuous welds longer than 12-inches, but less than 60-inches: No less than 1 weld in every 10 welds shall be tested; the length of each test shall be no less than 6-inches.
Continuous welds longer than 60-inches: 10% of the total weld length shall be tested, with no individual test being less than 6-inches. The initial test shall be performed at the center of the weld length. Either or both ends of the continuous weld shall be tested as needed to produce the required 10%. As needed, successive tests shall be performed along the length of the weld to produce the required 10%.
It would be great if the project specification includes the level of detail needed for clarity, but in the absence of detailed instruction, the inspector can propose an inspection plan to the Engineer for his review and approval.
Best regards - Al