When a vacuum box bubble test is to be performed on a large area such as a flat bottom, it would be good leak testing practice to:

test all the weld joints in one long sequence as fast as possible so as not to miss any areas.

test the large area by dividing it into small sections (one day or half day work for the test crew) rotating or changing testing personnel after testing of a section is complete in order to minimize the human fatigue factor.

allow the test operator a 15 minute break every hour

perform a hydrostatic test first in order to determine if vacuum box bubble test is even required.