The Engineering Authority (EA) determines this requirement. "Approval" by the EA is briefly discussed in 1.3.
I have worked in aerospace/aircraft off & on since the late 70's, this included several FAA repair stations. Having qualified to Pratt & Whitney, MIL-STD 5021, 1595, 2219 (Groups IA/IB, IIA/IIB, IIIA, IV, V and VI) for flight hardware and numerous AWS codes for ground suppport equipment. The NDT, mechanical or metallurgical testing was typically performed by a 3rd party test lab. I believe this was more of a contractual requirement. Check your contract docs.
Some companies permitted transfer of certifications if they were <6 months old (or new), while most required retesting each time you moved to a new company. D17.1 does not permit this practice per 4.2.2.
Critical power-plant components or ordnance sytems usually required testing every 6 months. Eye exams were often on a yearly basis.